Lighting for Healthcare Shift Workers

Regardless of shift worked, healthcare work in general carries significant risk for work absences due to illness, stress, and injury, especially among older workers (16, 17).

• Healthcare support workers have the highest work absence rates of any occupation (18).
• Healthcare practitioners and technicians have the highest work absence rates of any professional occupation (18)
• While rates are declining across all US work sectors, OSHA-recordable illness and injury rates for hospital workers still remain nearly twice as high as those for all private industry combined (19, 20).
• Injuries among healthcare professionals occur at almost three times the rate experienced by other professionals (21).

Shift work is an inescapable fact of life in the 24-hour operating environment of modern hospitals. To ensure continuity of patient care (e.g. efficiency, fewer patient handoffs) and accommodate nurses’ personal preferences (e.g., shorter work week, improved work-life balance) (22, 23), nursing schedules typically follow 12-hour shifts that can include working through the night (see graphic above), when alertness levels are the lowest and the pressure for sleep is the highest (see Sleep, accidents, and shift work). Nursing therefore carries an inherent risk for disruption of the circadian system, and nurses who work night shifts show elevated health and occupational safety risks compared to their colleagues who work days (24-26). It is reasonable to infer that the greater the irregularity of work and sleep, the greater the risk to health.

The research on risks posed to shift-working nurses shows:

• Based on a 22-year follow-up on almost 70,000 older (65–66 years old) nurses who took part in the first Nurses’ Health Study (27) beginning in 1976, mortality from all causes (especially cardiovascular disease) increased significantly among those working rotating shifts for 5 years or more compared to their colleagues who never worked nights (25).
• A more-recent review of data from over 114,000 participants in the second Nurses’ Health Study II (beginning in 1989) found elevated risk for breast cancer among nurses who had worked rotating shifts for 20 years or more (28), as supported by other studies (29).
• Generally, elevated risk for breast cancer is greatest among nurses who work night shifts (8), especially among those who have been exposed to rotating night-shift work at a younger age (28).
• Some recent research has cast doubt on claims of an association between working night shifts and breast cancer, however, so continued investigation has been recommended (30-32).

The risks are not limited to physical health. Nurses working longer shifts (i.e., >10 hours) are up to 2.5× more likely to experience burnout (i.e., emotional exhaustion, depersonalization of patients, lack of vitality) and job dissatisfaction compared to nurses working shorter shifts (33). Working shifts longer than 13 hours is associated with decreased well-being, increased job turnover, and greater patient dissatisfaction (33).

Sleep problems are common in healthcare workers, almost 50% of whom sleep fewer than 7 hours per 24-hour cycle (34), which places them among workers with the highest prevalence of short sleep durations for all occupations (35). Recent research examining shift-working nurses shows that day- and evening-shift nurses experience more regular and consistent rest–activity cycles compared to night-shift nurses (36). Nurses showing greater sleep fragmentation and/or more irregular rest–activity cycles also experienced lower daytime activity levels on workdays.

Poor sleep has consequences for health and emotional well-being. A 2018 long-term study of healthcare shift workers in four U.S. academic hospitals found that participants who reported sleep disorders on a screening questionnaire were almost twice as likely to experience adverse safety outcomes over the following 6 months and increased risk for anxiety or depression by almost two-thirds (37).

Most of the available research examining specific work-related injuries to nurses focuses on needle-stick and musculoskeletal injuries, with a number of those studies associating elevated risk for injury with shift work and night shifts.

• A 2007 study linked increased needle-stick injury risk with the amount of time worked per day and month, as well as with working evenings/nights and shifts longer than 13 hours (38).
• A 2012 study examining injured worker case files found increased risk for injury when nurses and patient-care associates worked two or more consecutive 12-hour shifts, especially when those shifts involved nights, compared to work shifts that were preceded by time off (39).
• A 2015 survey of 1,744 newly licensed registered nurses found that needle-stick injury risk increased 32% for those working overtime and 16% for those working nights (40).

Risks for shift workers do not end in the workplace. Because many nurses are sleep-deprived during their work week, the drive home after their shift can be particularly hazardous. These data are supported by the 2018–2019 Healthy Nurse Survey conducted by the American Nurses Association, which reports that 14% of nurses either nodded off or fell asleep while driving during the past 30 days (34).

• A landmark 1992 study of 635 Massachusetts nurses published in the American Journal of Public Health showed that, compared to those working either day shifts or evening shifts only, rotating-shift workers were twice as likely to fall asleep while driving home and experience near-miss car accidents (41).
• Nurses working night shifts are also more likely to have difficulty staying awake while driving home compared to nurses working other shifts (42, 43).
• A 2013 study using objective (rest-activity, eye movement) and subjective (questionnaires) assessments of nurses’ drowsiness and their driving performance on a controlled track following an 8–10-hour night shift found that the participants were far more likely to experience a hazardous driving event than on their drive to work before the shift, especially if they had been awake for 16 hours or more (44).
• A more recent 2019 Australian study employing similar methods confirmed this relationship between adverse driving events and the drive home after the night shift, finding that sleepiness-related events tended to occur after the first shift and inattention-related events tended to occur after subsequent shifts in the same cycle (45).
• The Australian study also found a connection between adverse driving events, the timing of the nurses circadian rhythms, and the time nurses had spent awake (45).

• Night-shift healthcare workers are clearly at higher risk for accidents and health problems associated with circadian disruption.
• In the short term, these health problems broadly resemble those associated with jet lag, such as sleep and digestive troubles, fatigue, and mood problems.
• Longer term night-shift work has been associated with chronic metabolic diseases (e.g., cardiovascular disease, diabetes, and obesity), various forms of cancer, mood disorders, and elevated risk for accidents both on and off the job.
• While further research is needed, there is a present growing urgency to address these problems in innovative ways.

The approach described in this web resource joins a growing body of research into non-pharmacological lighting interventions for treating depression, chronic pain, sleep problems, and even Alzheimer’s disease and related dementias (ADRD). These approaches have shown great promise for improving peoples’ lives without the undesirable side effects of conventional medications.

Light is not just for vision. While it has always been essential for seeing the world around us, light has also played a crucial role in our evolutionary adaptation to the natural 24-hour environment created by Earth’s rotation. With a few exceptions, most living things (including plants, insects, and even microbes) cycle through internal or endogenous biological rhythms that repeat daily. Called circadian rhythms, they include all of our metabolic, physiological, psychological, and behavioral processes. Human circadian rhythms are synchronized by the pattern of light and dark reaching the back of our eyes, and primarily regulated by a structure in the brain’s hypothalamus region called the master biological clock. (See Regulation of circadian rhythms)

This master biological clock interacts with and regulates a complex network of biological clocks that are found in almost all tissue and every organ in the body. Each clock has its own rhythmicity and responsibility for generating and regulating the timing of its own circadian rhythm on cues from the master clock (50, 51). Feeding and fasting, body temperature, blood pressure, heart rate, kidney function, immune system functions, and body temperature are just a few examples of the circadian rhythms regulated by these biological clocks. And the light received by our eyes’ retinas is the primary stimulus that makes everything tick at the right time (52).

Why do circadian rhythms even need to be regulated? In the absence of external or exogenous cues or zeitgebers (German zeit = time, geber = giver), human circadian rhythms free-run on a perpetually repeating cycle that is slightly longer than the solar day. These cycles run about 24.2 hours on average, with extremes of as long as 1 hour on either side of that average depending on individual differences between people (53).

Over time, without timing cues provided by the light and dark patterns of the daily solar cycle received at the eyes, our circadian rhythms and bodily processes would eventually cycle out of sync with the local time of our variable, seasonal environment. We would go to bed later and wake up later, have difficulty getting to work on time and feeling alert once we got there, become hungry outside regular mealtimes, and feel fatigue in after-work social situations. In other words, we would tend to experience the common things of everyday life at the wrong time of day relative to our work schedules, social and family commitments, and biological needs.

Scientists who study circadian rhythms — known as chronobiologists — measure the timing of our bodies’ cyclical processes, which can be tracked, for example, by monitoring the body’s release of hormones like melatonin or cortisol. When our circadian rhythms cycle on a later schedule than they should, as indicated by the later timing of the body’s release of nighttime production of melatonin, for example, our circadian rhythms are said to be delayed. In this case, we tend to fall asleep later in the evening or into the early morning and wake up later. Conversely, when our circadian rhythms cycle on an earlier schedule than normal, leading us to fall asleep and wake up earlier, our circadian rhythms are said to be advanced. (See Hormones, sleep, and alertness.)

Depending on the time of day and the characteristics of any light exposures that we might encounter, light can either delay or advance the timing of our circadian rhythms (see Light and the circadian system). This is basically what happens when we cross multiple time zones and experience jet lag, which becomes pronounced as we grow older and when traveling east (54). Regardless of the direction traveled, with the new destination comes a different schedule of light and dark that is at odds with the one we left at home, running either a few or even many hours too early or too late.

Jet lag results in disturbed sleep and daytime fatigue, generalized feelings of being under the weather, and difficulty in concentrating because our circadian systems have not yet adjusted to the new environment (55). But those symptoms gradually subside as our circadian system becomes synchronized or entrained to the new local environment by adapting to our destination’s local solar cycle. Social activities and shift work that might fall outside the regular sleep/wake cycle, producing a similar effect, are known to chronobiologists as social jet lag (56) and shift work disorder (14, 57) (see Circadian entrainment and disruption).

Four key factors related to light-dark environmental cues influence circadian entrainment:

• The amount or level of light received at the eyes (“bright or dim?”)
• The spectral properties of the light experienced, either short-wavelength (“cool” or “bluish”) or long-wavelength (“warm” or “reddish”) light
• The timing and duration of light exposures (“when, and for how long?”)
• A person’s cumulative history of light exposures (“how much, what spectra, and when over the past 24 hours?”).

These lighting characteristics work together to affect the circadian system, either entraining or desynchronizing our circadian rhythms. For example, healthy people who follow typical daytime work schedules require high levels (amount) of short-wavelength (spectrum) light for at least 30 minutes (duration) in the morning (timing) to promote circadian entrainment, followed by exposure low levels (amount) of long-wavelength (spectrum) light in the evening (timing). This consistent daily pattern of light exposures (history) promotes regular bedtimes, good quality sleep, and optimal wake times for the day ahead. Due to various factors such as age, individual differences between people, and varying long-term histories of light exposure (e.g., permanent night-shift workers or “night owl” chronotypes), people can be differentially sensitive to light and thus will have differential responses.

While these variable factors preclude a “one-size-fits-all” lighting prescription for circadian entrainment and health, this web resource provides some useful general guidelines for the benefit of almost anyone. (See Lighting for alertness, performance, and health.)

The human master biological clock (SCN) in the hypothalamus is activated when light received by the retinas is converted into neural (i.e., electrochemical) signals in a process known as circadian phototransduction, which employs a photic neural pathway known as the retinohypothalamic tract (RHT) that extends from the retinas to the SCN. The retinal cells involved in phototransduction include the well-known rod and cone photoreceptors as well as the intrinsically photosensitive retinal ganglion cells (ipRGCs), which were identified only at the turn of this century (58-60). Working largely independently of image formation, the ipRGCs have been documented to be active even in totally visually blind animals and humans (61, 62).

Well-timed and carefully specified light exposures can promote the entrainment of our circadian systems (see Light and the circadian system). Exposure to light at the wrong time, or not receiving enough light at the right time, has become increasingly common since the advent of electrical lighting over a century ago. Exposure to light at night (LAN), and even a complete reversal of the day–night pattern in the case of night-shift workers, have become facts of life in our 24-hour society. Exposure to LAN and insufficient exposure to light early in the day has been linked with disruption of the circadian system (64). Long-term disruption of the daily cycle of light and dark can lead to chronic disruption of the circadian system, which has been associated with metabolic (leading to weight gain, obesity, and type 2 diabetes) and cardiometabolic dysregulation, certain forms of cancer, depression, and other maladies (10, 65, 66). Although the precise reasons for this association are not yet clear, a growing body of research suggests that this disruption might be related to lighting conditions in the built environment and irregular exposures to daylight associated with our around-the-clock lifestyle.

Compared to regular daytime workers, nurses who work during or through the night have very different needs that depend on how they choose to adjust to their work schedules. As detailed in Lighting solutions for alertness, performance, and health>, some consider it desirable to adapt to a nighttime schedule by becoming more or less nocturnal, whereas others prefer to maintain an adjusted daytime-active schedule that permits them to engage in family, social, and community activities. Whatever path is chosen, those working at night need to regulate their exposure to high levels of light while at work, especially blue light, while also being careful to receive sufficient light to promote alertness and perform specific critical visual tasks like inserting an IV cannula.

Scientists at the Light and Health Research Center (LHRC) at Mount Sinai have been exploring the role that light plays in synchronizing the circadian system, and how exposures to the right kind of light at the right time can help to avoid these negative outcomes. Our research to date among diverse populations such as office workers, submariners, and older adults with Alzheimer’s disease suggests that appropriately timed, tailored lighting can improve measures of alertness, depression, mood, and feelings of vitality. The research funded by this project (NIOSH 5R01OH010668-03) indicates that similar lighting solutions can also be applied to shift-working healthcare personnel (see Red light: A novel, non-pharmacological intervention to promote alertness in shift workers).

• Most living things cycle through natural circadian rhythms that repeat daily and include all of our metabolic, physiological, psychological, and behavioral processes.
• Light received by our eyes’ retinas is the primary stimulus that makes the biological clock tick.
• The circadian system is stimulated by the amount of light received at the eyes, the light’s spectral properties, the timing and duration of exposure, and our personal history of light exposures.
• Shift work can disrupt circadian rhythms, but lighting can be used to counter the disruption and promote good sleep, healthy outcomes, and emotional well-being.

Light is not just for vision and stimulating the circadian system. Light also exerts an acute alerting effect on humans that is similar to the boost provided by a cup of coffee. Yet as we already know from Light and circadian rhythms, light’s alerting effect can come at a cost to circadian entrainment, especially when experienced at the times we might need it the most, like during the night shift. Depending on the amount, spectral properties, and our history of exposure, light at the wrong time can also disrupt our circadian system and thereby threaten our health and well-being.

The human body’s central nervous and endocrine systems transmit information throughout the body to regulate all bodily functions. The nervous system is essentially a hard-wired network made up of over 100,000 million nerve cells that instantaneously transmit electrical signals from the body’s peripheral nervous system and the outside world to the central nervous system (i.e., the spinal cord to the brain), which in turn dispatches electrical signals to direct the actions of the body’s muscles, organs, and tissues. The more slowly responding endocrine system, on the other hand, sends chemical signals through the bloodstream in the form of hormones excreted by the body’s endocrine glands to regulate various processes (e.g., heart rate, blood pressure, appetite, body temperature), including the sleep/wake cycle.

As discussed in Light and the circadian system, the secretion of the hormones melatonin and cortisol both follow circadian rhythms that are regulated by inputs from the master biological clock based on cues provided by the patterns of light and dark received at the retinae (67).

Melatonin, known as the “darkness hormone” because it is released at night and under conditions of darkness, is produced by the pineal gland. In daytime-active species, melatonin signals to the body that it is time for sleep (68). The concentration of melatonin in the bloodstream begins to diminish in the latter hours of sleep and after waking remains barely detectable throughout the day.

Cortisol, produced by the adrenal glands, promotes the synthesis and storage of glucose and influences metabolic, immune, muscle, and functions (69) as well as various brain activities as cognitive function (70) and the regulation of emotions (71). In humans, cortisol remains in low concentration in the bloodstream throughout the day but gradually elevates throughout the night, culminating 30–45 minutes after waking in a sharp increase known as the cortisol awakening response (72, 73). Although our understanding of cortisol and its response to light are not as well understood as that of melatonin (74, 75), it has been speculated that the cortisol awakening response promotes alertness and arousal in anticipation of the forthcoming day’s demands (76-78).

The effects of light on alertness at night are well documented (79-82) and have been associated with light’s ability to suppress melatonin (83), particularly when the source is short-wavelength (“cool” or “bluish”) light that matches the 460-nm peak spectral sensitivity of the human circadian system (84-86). Studies using the kind of white light typically found in workplaces have shown that very high light levels (> 2500 lux compared to about 200 lux during the daytime in hospital corridors) are needed to promote subjectively and objectively assessed measures of alertness (see Light, alertness, and task performance). On the other hand, much lower levels (30 lux) of short-wavelength light can produce a similar alerting effect (86-88). These studies suggest that melatonin suppression may play a role in alertness by “fooling the body” into thinking that it is daytime and therefore time to be awake and alert.

There’s a catch, though. Studies examining light’s effects on alertness have also been conducted during the daytime, when the body’s melatonin levels are so low the hormone is virtually absent (89-91). These studies are supported by pioneering studies measuring brain activity (via functional magnetic resonance imaging [fMRI]) in response to light, finding that both bright (> 7000 lux) white light and lower levels (7.5 lux) of short-wavelength (peak close to 473 nm) light more effectively activated brain regions associated with alertness compared to dim light or higher levels (24.5 lux) of middle-wavelength (peak close to 527-nm) light (92, 93). These and other studies suggest that melatonin suppression is indeed not required to elicit alertness (94, 95).

Clearly, it appears that processes other than melatonin suppression are involved in light’s alerting effect, and that those processes are instantaneous and probably related to the central nervous system rather than just the more slowly moving endocrine system (96).

Central to the work performed under our NIOSH grant, a new line of study has shown that long-wavelength (peak close to 630 nm) “warm” or “red” light can elicit alertness, both night (47, 97) and day (88, 98). A key benefit of using red light to promote workplace alertness, especially among personnel who work irregular schedules involving nighttime hours, is that red light’s peak wavelength (close to 630 nm) does not overlap with the peak spectral sensitivity of the circadian system. In other words, red light can promote alertness with no ill effects on circadian rhythms and, by extension, workers’ sleep, heath, and general well-being.

The circadian system’s master biological clock helps to regulate the sleep–wake cycle in tandem with a process known as sleep homeostasis (99-102) that maintains a constant balance between sleep and wakefulness. The circadian system essentially regulates the timing of sleep while homeostasis regulates the need or pressure for sleep, in what has been conceptualized by sleep scientists as a two-process model (102, 103). Following what chronobiologists call Process C, the circadian system promotes the timing of sleep at roughly the same time every day and orchestrates physiological process like the secretion of melatonin (see Light and circadian rhythms).

Sleep homeostasis follows Process S, which builds pressure for sleep during waking hours and dissipates pressure when we sleep, even if only during a short afternoon nap. The interplay between these two systems has been characterized by one sleep researcher as a competition, with the “winner” determining whether we are awake or asleep (104). Experiments using animals whose circadian rhythms have been disrupted by surgical lesions on the SCN (i.e., the master biological clock) have shown that Process S is not affected, suggesting that the two processes are independently regulated despite whatever “crosstalk” might occur between them (105).

The workings of the two-process sleep model should be familiar to anyone who has experienced sleepiness during the conventional workday. Around 2–4 PM, or 16–18 hours after the previous night’s bedtime, many people experience a decline in alertness and performance known as the post-lunch dip (106). At this time of day, Process C cannot completely counteract Process S, which is gathering momentum prior to its abrupt peak and decline about 8 hours later. Process C reaches its peak early evening (also known as wake maintenance zone), when falling asleep is very difficult (some refer this time as their “second wave”). After levelling off and reaching its peak, Process C switches and resumes sending the body a sleeping signal. Combined with the high amount of sleep debt accumulated by Process S throughout the day, Process C finally triggers sleep. (The LHRC has innovated a unique application for getting us through the post-lunch dip by using light to promote acute alertness; see Lighting for alertness, performance, and health).

Important for the purpose of this website, light also has dynamic relationships with alertness, sleepiness, and the sleep–wake cycle. As noted, light elicits an acute alerting response both day and night, in which “acute” denotes a rapidly occurring, temporary effect that is more or less independent of the circadian system and does not necessarily influence the circadian system’s timing (107). On the other hand, light can also influence the timing of sleep–wake Process C. Receiving light of a particular spectrum (especially short-wavelength light) in sufficient levels for a sufficient duration and a particular time of day can affect the timing of when we sleep, which if experienced for a sustained period of days or weeks can lead to circadian disruption and its negative consequences (see Circadian entrainment and disruption).

Alertness and sleepiness are like two sides of the same coin, each with its own distinct domain. In daytime-active species, alertness is strongly associated with the daytime as well high levels of environmental awareness (97) and high levels of sensitivity to external stimuli (108), whereas sleepiness is generally associated with the nighttime and inverse levels of both states. This association creates obvious problems for those who work through the night.

For experimental purposes, alertness and sleepiness are also measured using complementary methods such as subjective measures (e.g., self-reports like the Karolinska Sleepiness [KSS] scale (109) and Subjective Vitality Scale (110)); objective measures of task performance in attention (e.g., Psychomotor Vigilance Task (111)) versus executive function (e.g., Sustained Attention to Response task (112) and N-back task (113)), and objective physiological outcomes (e.g., brain activity measured via electroencephalography [EEG]).

These outcomes have been used to assess light’s alerting effects in a broad range of studies whose findings have significant practical applications for healthcare personnel who work through the night. Studies to date, however, have mainly focused on the acute alerting effects of light in daytime workers, particularly with respect to providing daylight in workplaces.

In a 2018 review of studies testing the effects of white light on alertness, Lok and colleagues (96) found:

• Self-reported alertness was improved by morning and/or afternoon bright light (750–5000 lux) exposures of varying duration (30–90 minutes) in 14 of the 19 studies examined.
• Improvements in performance testing outcomes in eight of the 19 studies examined, with three studies reporting negative results.
• Physiological indicators (central nervous system activity [EEG] and autonomic nervous activity [e.g., pupil dilation, eyelid movement, heart rate variability]) of alertness in six of the 19 studies.

Field research conducted by the Figueiro and colleagues (114, 115) delivering circadian-effective lighting to workers in government office building settings (General Services Administration and United States Embassies), again during the daytime, found:

• Self-reported sleepiness scores (KSS) were reduced by the study’s intervention after arrival in the workplace compared to baseline measures, with a statistically significant improvement during the post-lunch dip. (The asterisk in the graph below denotes a statistically significant difference and the error bars represent standard error of the mean.)

• Self-reported vitality scores recorded at four localities (two office buildings and two northern latitude embassies) were significantly improved by the end of the study’s three-day intervention period. (The asterisks in the graph below denote statistically significant differences and the error bars represent standard error of the mean.)

Finally, preliminary results from a field study by Figueiro and Pedler (116) found that personal light glasses delivering red and blue lights to day-shift and night-shift nurses’ eyes while working improved response times in performance tests, with the red light improving response times both day and night (see Additional Information, Red light: A novel, non-pharmacological intervention to promote alertness in shift workers). Importantly, only the blue light suppressed the nurses’ nighttime melatonin levels, illustrating the value of red light as an alerting stimulus that only minimally affects melatonin levels and thus the circadian system.

• Light exerts an acute (i.e., rapidly occurring) alerting effect on humans that is similar to the boost provided by a cup of coffee.
• Light has the ability to suppress the hormone melatonin, particularly when the source is short-wavelength (“cool” or “bluish”) light that matches the 460-nm peak spectral sensitivity of the human circadian system.
• Very high levels of white light are needed to promote alertness, while much lower levels of short-wavelength (“blue”) light can produce a similar alerting effect.
• New research shows that long-wavelength (peak close to 630 nm) “red” light can also promote alertness, both night and day.
• Red light can promote alertness among those working irregular schedules involving nighttime hours with no ill effects on their circadian rhythms and, by extension, their sleep, heath, and general well-being.

Scientists at the Light and Health Research Center (LHRC) have been actively researching light’s effects on alertness, performance, and human health since the center’s inception in 1988, exploring these effects and practical solutions for improving the lives of diverse populations. In the course of this research, the LHRC has developed the circadian stimulus (CS) (118-120) metric, which is based on the spectral sensitivity of the retinal phototransduction mechanisms stimulating the response of the master biological clock (see Circadian entrainment and disruption). The basics of CS are explained in a freely available online video series created by LHRC researchers.

Based on nocturnal melatonin suppression after a 1-hour exposure to light, CS is expressed on a relative scale from a threshold value of 0.1 (about 10% melatonin suppression), indicating minimal stimulus of the circadian system, to a maximum saturation value of 0.7 (about 70% melatonin suppression), indicating maximal stimulus. In other words, the higher the CS value, the greater a light source’s effect on the circadian system. (More information on this topic can be found at the Lighting for Healthy Living website, which provides detailed information on lighting design suggestions for healthcare personnel, patients, and visitors in its background section.) For detailed information on the CS metric and how the CS levels can be calculated in the built environment, please feel free to refer to the LHRC’s freely available online CS Calculator.

If the CS metric sounds overly technical and beyond the reach of the average person, it certainly doesn’t need to get in the way of busy non-experts who are interested in using light to promote workplace alertness, health, and well-being. Fortunately, there are also much more straightforward rules of thumb for managing light exposures while working shifts, which are explored below.

Although the lighting requirements of hospital work (e.g., visibility and alertness for task performance) remain constant around the clock, the needs of healthcare workers themselves can vary widely throughout the day. It is therefore important that frontline 24-hour workers like nurses be mindful of making adjustments to their lighted environment, wherever and whenever possible, to balance the needs of the tasks at hand and those of their circadian systems.

Of the four characteristics of light affecting the circadian system (see Light and the circadian system), it is perhaps easiest to adjust both the timing and duration and the personal history of our 24-hour light exposures. This is especially relevant for those working day shifts, who have greater control over their nighttime lighted environment and can take the following steps to achieve circadian entrainment and it benefits (improved sleep, alertness, mood):

• Sit next to an unshaded window to receive daylight soon after waking for at about 30–60 minutes in the morning.
• Aim for high levels of light (> 300 lux at the eye) through the early and mid-day by using a supplemental lighting device (see below) or seeking daylight from windows whenever possible.
• Spend time outdoors on your lunch break, if possible, even going for a brief walk if you have time.
• Minimize exposure to high levels of light (> 30 lux) in the evening (2 hours prior to bedtime) and through the night by dimming your bedroom and bathroom vanity lighting. Use lowly positioned nightlights to illuminate the floor line and doorways between your bedroom, bathroom, and kitchen for nighttime navigation.
• Avoid exposure to self-luminous electronic devices (e.g., cellphones, tablet computers, etc.) in the evening, at least 2 hours prior to bedtime.
• Avoid excessive outdoor exposure to daylight during the late afternoon and early evening in the summer months, or wear orange-tinted eyeglasses that filter out short-wavelength “blue” light (> 525 nanometers).

When at work, it is generally much more difficult for individuals, especially nurses, to adjust the remaining two characteristics of light affecting the circadian system (see Light and the circadian system), amount and spectrum, because they have far less control over their workplace’s nighttime lighted environment. This is largely because strict lighting specifications with respect to amount (or light levels) are recommended for hospitals and healthcare facilities by organizations like the Illuminating Engineering Society (121), and few if any choices with respect spectrum are available in most facility-wide lighting systems.

For these reasons, the LHRC recommends adopting a portable personalized lighting approach, specifically when aiming to promote and circadian entrainment while avoiding circadian system disruption. While not yet widely available as viable commercial products, new technologies using desktop luminaires (i.e., lamps) and wearable light goggles emitting blue, white, or red light hold great promise. Broad guidelines for their use are provided in the table below.

Hospitals provide around-the-clock patient care, compelling health care professionals to work evening, overnight, and rotating shifts. The demands of this 24-hour work schedule are inherently out of sync and conflict with workers’ physiological demands as well as their social and family commitments, which can interfere with alertness and circadian entrainment and thereby compromise worker safety, job performance, sleep, mood, and health.

Prior to making any decisions about their personal 24-hour lighted environment, shift workers first have to assess their work schedule and then decide how they plan to adapt to it, asking questions like:

• How long does the shift schedule run?
  • Only two or three consecutive night shifts, for example, would minimize the number of days spent in a desynchronized state per week (122).
  • A greater number of consecutive shifts, on the other hand, would provide workers with increased opportunity to become better entrained to their work schedule (123).
• How long is each shift?
  • Working longer shifts (10–12 hours) or overtime shifts (as many as 16 hours) will maximize workers’ time off the job, perhaps making entrainment to the night shift unnecessary.
• Is it possible to take a nap on the job?
  • Evidence shows that taking naps while working can reduce subjective sleepiness (124) and promote alertness (125, 126), though caution must be taken to counter the effects of temporary sleepiness (known as sleep inertia) after waking (127, 128).
• Can I take a nap at home before I report for night shift?
  • Studies have linked napping at home before night shifts with reductions in accidents (129); improved performance while at work without affecting the quality of daytime sleep (130); and improved measures of objectively assessed alertness, especially when combined with caffeine (131).
• Should I align my sleep cycle with my time off, or do I feel better when my sleep cycle is better aligned with my time on the job?
  • It depends. If you are not a permanent night worker, aligning the sleep cycle to their time off provides greater opportunities to fulfill social and family commitments. On the other hand, it will certainly be harder to stay awake during the night shift.

Depending on how you have resolved these questions, a few key lighting principles are important to remember:

• Maintaining a regular, robust 24-hour pattern of light and dark at the eyes is crucial for circadian entrainment and its attendant benefits, whether at work or rest.
• While high light levels (i.e., amounts) promote alertness at any time of day or night, because that same light can also stimulate the circadian system it is desirable to weigh alertness against the benefits or detriments of receiving circadian stimulus at any given time.
• Because the circadian system is maximally sensitive to short-wavelength (blue) light, even low levels (e.g., 30 lux at the eyes) of a saturated blue light can deliver more CS than much higher levels (300 lux at the eyes) of cool white (4000 K) light.
• Blue or bright light is generally desirable early in the daytime, but can disrupt the circadian system if experienced at night.
• Stimulating the circadian system with blue or high levels of white light at night promotes alertness but also suppresses the body’s production of melatonin (see Circadian entrainment and disruption) and disrupts the circadian system.
• The daytime sleep period following the night shift is driven by sleep pressure (see Sleep/wake and acute alertness) rather than our natural circadian rhythms (e.g., melatonin levels) that cue the body to sleep at night.
  • Avoiding or minimizing light stimulus (such as bright morning light) by wearing sunglasses during the commute home can make it easier to fall asleep.
  • Drawing the window shades, or even installing blackout shades, in living areas and your bedroom would also be helpful.

Before the shift: Daytime preparation for work

• Upon waking in the afternoon, seek bright light (CS of at least 0.3) until reporting for your shift.
  • Sit outside or go for a walk if the weather permits
  • Eat meals in a room with lots of windows
• Take a nap in the late afternoon or early evening if you feel tired, or rest quietly if you aren’t ready for sleep.
• If not taking a nap, spend some time in bright light before departing for work, preferably outdoors if the season permits.

During the shift: Promote alertness

• At the beginning through the first half of the shift, use personal lighting devices to deliver high levels of white light (at least 300 lux at the eyes, CS of about 0.3) to delay your regular day-shift sleep time, while not fully disrupting the circadian system.
  • Desktop luminaire (blue or white light setting)
  • Light goggles (white light setting)
• To avoid disrupting the circadian system, maintain low light levels (< 30 lux at the eyes, CS < 0.1) until at least the middle of the shift, if possible.
  • Dim the brightness of self-luminous digital displays (e.g., computers, tablets, cellphones, etc.)
  • Use task lighting when viewing paperwork or tending to patients (e.g., administering medication, IV insertion, charting, etc.), if possible
  • Turn off selected overhead lighting to reduce overall light levels in nurses’ stations
  • Use orange-filter glasses to filter short-wavelength (blue) light from the lighting systems that cannot be turned off
• To promote alertness after “lunch” during the second half of the shift, use personal lighting devices to deliver at least 100 lux of red light or 300 lux of white light at the eye in rest areas or workspaces. Red light can be used intermittently throughout this period to provide an alerting effect similar to a cup of coffee without disrupting the circadian system.
  • Desktop luminaire (red light setting)
  • Light goggles (red light setting)

After the shift: Commuting and preparing for daytime sleep

• When winding down from the shift, either during the morning commute or when relaxing at home, avoid high light levels as much as possible.
  • Wear orange-tinted glasses to filter out short-wavelength light from morning sunlight if taking public transportation or walking
  • Wear dark sunglasses to block bright light from morning sunlight if driving home
  • Safety is your first concern, whether on the drive home or navigating spaces once at home

Rotating shifts, especially when they follow three “turns” (i.e., days, afternoon/evenings, and nights) that involve working through the night, can be difficult to adapt to and pose formidable challenges for circadian entrainment and health (see Risks of healthcare shift work>). Choosing whether to adapt to the new week’s work schedule or remain synchronized to the day shift can also be problematic, because daytime work is scheduled for every third shift turn rather than every other shift turn. While it has long been thought that rotating the three turns in clockwise order (i.e., days to afternoon/evenings to nights) is preferable because delaying sleep times is easier than advancing them (132), most workers rarely have a direct say in the scheduling of shifts (133).

Given the high variability of rotating schedules, quickly emerging contingencies (e.g., last-minute requests to work overtime), and biological differences between individuals, it is nearly impossible to specify facility-wide lighting that will accommodate the precise needs of all workers. Furthermore, as concluded in a recent review of 27 studies exploring lighting interventions to improve circadian adaptation to nighttime working hours, a lack of long-term data on the health outcomes of these interventions precludes making anything but broad recommendations (134). Fewer than half of the studies were conducted in the field with workers in real-world situations, only one field study included workers transitioning between day and night shifts (with non-significant objective results for the 30-minute bright light [10,000 lux] intervention), and none were randomized, controlled clinical trials.

In addition to the techniques shown above for night-shift workers (especially after the shift), some broad suggestions for managing personal light exposures might be of interest to rotating-shift workers.

• Those working rotating 12-hour shifts, for example, should not attempt full adaptation to the night shift, since both schedules permit daytime functioning equally well and a day-shift schedule is more compatible with personal and social commitments.
• Instead, light exposures can be used to ease the transition between schedules in preparation the upcoming shift sequence and mitigate problems with sleep and alertness.

For example, a Norwegian study of night-shift oil rig workers found that 30-minute bright light (10,000 lux) exposures experienced at 2 PM on the first transition day to a day-shift schedule—followed by bright light exposures delayed at 2-hour intervals over succeeding days—reduced workers’ perception of the time required to re-adapt to day shift (135). As indicated, however, more research on lighting interventions for rotating-shift workers is needed before precise light dosing specifications and schedules can be recommended.

• The CS metric can be used to assess and predict the circadian effectiveness of light sources and their influence on the human circadian system, with greater CS values indicating greater effects.
• Frontline 24-hour healthcare workers like nurses should be mindful to make adjustments to their lighted environment, wherever and whenever possible, to balance the needs of the tasks at hand and those of their circadian systems.
• Shift-working nurses should assess their work schedule and decide how they plan to adapt to it, asking questions like:
  • How long does the shift schedule run?
  • How long is each shift?
  • Is it possible to take a nap on the job?
  • Can I take a nap at home before I report for night shift?
  • Should I align my sleep cycle with my time off, or do I feel better when my sleep cycle is better aligned with my time on the job?
• Maintaining a regular, robust 24-hour pattern of light and dark at the eyes is crucial for circadian entrainment and its attendant benefits, whether at work or rest.
• While high levels (i.e., amounts) of light promote alertness at any time of day or night, that same light can also stimulate the circadian system, so it is desirable to weigh alertness against the benefits or detriments of receiving circadian stimulus at any given time.
• Because the circadian system is maximally sensitive to short-wavelength (blue) light, even low levels (e.g., 30 lux at the eyes) of a saturated blue light can deliver more CS than much higher levels (300 lux at the eyes) of cool white (4000 K) light.
• Blue light is generally desirable early in the daytime but can be disrupt the circadian system if experienced at night.
• To promote alertness and avoid disrupting the circadian system, red light (at least 100 lux at the eye) delivers negligible CS (< 0.01) and can be used to provide an alerting effect similar to a cup of coffee.
• Personal lighting devices can be used to deliver appropriately timed blue or red light, independent of facility-wide lighting systems.
• More field research is needed on the efficacy and long-term effects of lighting interventions to improve circadian adaptation to working hours.

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Chronotype
Classification describing the tendency of one’s natural sleep-wake cycle to occur early (“early bird” or early riser) versus late (“night owl” or late riser) in a typical 24-hour cycle.

Circadian entrainment
Synchronization of the body’s circadian system to the 24-hour solar day and one’s local time on Earth.

Circadian Light (CL_A)
Irradiance weighted by the spectral sensitivity of the retinal phototransduction mechanisms stimulating the response of the biological clock, based on nocturnal melatonin suppression after a 1-hour exposure to light.

Circadian stimulus (CS)
To quantify how different amounts, or levels, of CL_A affect the neural signal strength reaching the master biological clock, the circadian stimulus metric, or CS, was developed. CS represents the operating characteristic of the phototransduction circuits in the retina, from threshold (i.e., just barely enough to stimulate the master biological clock) to saturation (i.e., the maximum possible response to circadian-effective light, no matter how much optical radiation is provided to the retina).

Circadian system
The body’s endogenous timing system that aligns its physiology and behavior to the 24-hour solar day and local time on Earth. It includes the master biological clock, peripheral clocks that exist in every organ and most cells, and feedback-loops that generate oscillations in gene expression.

Cortisol awakening response
The change in cortisol concentration that occurs in the first hour after waking from sleep.

CS metric
A transformation of circadian light (CL_A>) into a relative scale from approximately 0.1 (10%), the threshold for circadian system activation, to approximately 0.7 (70%), response saturation. (Same as circadian stimulus.)

Endogenous
Caused, produced, or synthesized within or inside an organism or system. (See exogenous.)

Entrainment
To be drawn along with or following after. From the transitive verb to entrain.

Exogenous
Caused, produced, or synthesized external to or outside an organism or system. (See endogenous.)

Functional magnetic resonance imaging (fMRI)
A non-invasive technique for measuring and mapping brain activity by tracking minute changes in blood flow.

Illuminance
The amount of light (or luminous flux) incident on a surface. Illuminance is measured in footcandles (lumens/square foot) or lux (lumens/square meter). One footcandle equals 10.76 lux.

Jet lag
A temporary sleep problem experienced when quickly travelling across multiple time zones, resulting from a discrepancy between the destination’s time of day and the timing of one’s habitual sleep/wake cycle and circadian rhythms.

Master biological clock
Located in the brain’s hypothalamus above the optic chiasm, the master biological clock is composed of a cluster of cells (called the suprachiasmatic nucleus [SCN]) that receives direct input from the retina and generates circadian rhythms. It is the mammalian brain’s principal circadian pacemaker.

Night owl
A chronotype indicating someone who is late to bed and late to rise. (See Chronotype.)

Post-lunch dip
An early to a mid-afternoon decrease in physical and mental performance, accompanied by an increase in sleep propensity.

Process C
A daily alerting rhythm that is generated by the master biological clock. It interacts with Process S in the sleep/wake cycle.

Process S
In the sleep/wake cycle, the pressure for sleep that accumulates through the day and reverses at night during sleep. It interacts with Process C.

Shift work disorder
A cluster of physical and temperamental problems associated with the misalignment between the habitual diurnal sleep/wake cycle and irregular/changing work schedules. Symptoms include sleepiness when alertness and productivity are required, insomnia and other sleep problems, difficulty concentrating, lack of energy, and emotional and social difficulties.

Sleep inertia
The transitional state between sleep and wake characterized by impaired performance, reduced vigilance, and the desire for sleep. Its effects can last between a few minutes to as long as several hours.

Social jet lag
The discrepancy between the timing of circadian rhythms and social activities/commitments. Calculated as the difference between the measured midpoint of sleep on workdays and free (non-working) days.

Sympathetic nervous system
One of the two divisions of the autonomic nervous system (i.e., the part of the peripheral nervous system that is not under conscious control), the sympathetic nervous system innervates the heart and blood vessels, sweat glands, viscera, and adrenal medulla.

White light
A light source with a very broad optical bandwidth, typically categorized in terms of its position along the blackbody radiator and correlated color temperature (CCT), report in the unit kelvin (K).

Zeitgeber
From German, meaning “time-giver.” Zeitgebers are environmental or behavioral cues that synchronize or entrain circadian systems to the solar day. Light is the most effective zeitgeber for the human circadian system. Other zeitgebers include eating and drinking patterns, activity or exercise, social interactions, and air temperature.