Circadian Rhythms and Shift Work

This Policy Resource and Education Paper is an explication of the Policy Statement Emergency Physician Shift Work,


Emergency medicine is known as a high stress specialty. The adverse effect of constantly rotating shifts is the single most important reason given for premature attrition from the field.1 The problems of rotating shifts stem mainly from working in opposition to the body's normal circadian rhythms. The major circadian rhythm involved is the sleep/wake cycle. Social isolation of those who must work while others sleep is also a major problem.

There are many biological and social problems associated with rotating shifts. Physical problems include an incidence of peptic ulcer disease eight times that of the normal population. Cardiovascular mortality has also been noted to be increased among shift workers. One author estimates that the risk of working rotating shifts approaches that of smoking one pack of cigarettes per day.2 Other physical problems include chronic fatigue, excessive sleepiness, and difficulty sleeping. Part of the social toll on those who must work rotating shifts is reflected in an increased divorce rate. Shift workers are also known to have higher rates of substance abuse and depression. Shift workers are much more likely to view their jobs as extremely stressful. Accidents are increased as a result of working shifts. All of the recent major disasters attributed to human error (Exxon Valdez oil spill, Three Mile Island, Bophal chemical plant explosion, Chernobyl), occurred on the night shift, when alertness is at its lowest point.

Circadian Rhythms

Circadian comes from two Latin words circa: about and dia: day. It refers to the bodily rhythms that vary throughout the day in a periodic fashion. These rhythms have been recognized since the times of Aristotle and Hippocrates. Many bodily functions exhibit circadian rhythms, from the best known sleep/wake cycle to all of the vital signs. As we become capable of more precise measurements, more and more circadian cycles are being recognized. Even bone length has been found to exhibit a circadian periodicity.

Most circadian rhythms have both an endogenous component (regulated by an internal clock located in the supra-chiasmatic nucleus of the hypothalamus) and an exogenous component.3 The exogenous component is composed of various time clues called zeitgebers. One of the most powerful zeitgebers is the light/dark cycle.

Temperature is one of the best studied of the circadian rhythms and exemplifies the effect of endogenous and exogenous factors. People with a diurnal orientation have a peak temperature about 4 pm and a trough about 4 am. During the day people are up using muscles, generating heat, and eating which produces heat as food is metabolized. During the night not only is muscle use decreased, but one doesn't eat. Subjects kept in a so called steady state, forced to remain in bed but awake and fed the same amount of calories each hour still exhibit the same temperature curve, but not with as much periodicity (peaks and troughs do not differ by as much). In normal individuals, the endogenous and exogenous components of the circadian rhythms tend to complement each other and work in harmony. Those who must work at night and sleep during the day pit the endogenous and exogenous factors against each other and suffer the consequences. One important finding about the internal "clock" is that it runs on a 25-hour day, not the expected 24-hour day. Subjects who are isolated and removed from all zeitgebers will predictably go to bed an hour later each "day" and sleep an hour longer into the next day. Why this is so is unknown. It is postulated that this allows for external adjustment depending on the season and other external considerations. This 25 hour day explains why it is so easy to stay up later during holidays but so hard to get back on a work schedule rising earlier. It is also why traveling from east to west is much easier (where one adapts by staying up later and sleeping in) than vice versa. This is the basis for recommending a clockwise shift rotation that takes advantage of this natural tendency to stay up later and later.

Different circadian rhythms will change at different rates and with greater difficulty. This accounts for the jet-lagged feeling as one attempts to change to a nocturnal schedule and vice versa. It has been compared to an orchestra that starts with one conductor playing in harmony but when another conductor appears with a different beat there will be cacophony as the pieces convert at different rates to the new maestro. This jet-lagged feeling while the different rhythms adjust at different rates is the inevitable result of attempting to reset ones bodily clock to a nocturnal schedule. It is referred to as circadian disharmony.


In order to understand the effects of shift work and how to best schedule any 24-hour operation, some understanding of sleep is necessary. Little enough is known about normal sleep taken at night and even less is known about the sleep of those who must attempt to sleep in the day when years of evolution dictate otherwise. It is not clear how much sleep is actually necessary for optimal health. There is evidence that very long natural sleepers and very short sleepers have increased mortality.

Sleep is divided into several stages based on electroencephalogram (EEG) criteria. Stage I is the initial part of any sleep episode lasting 10 to 15 minutes. Most subjects when awakened from stage I will deny being asleep at all. Stage II accounts for the largest percent of sleep (50%), yet it is the least understood of all sleep stages, because it is the matrix from which all the other stages proceed. Sleep stages are typically studied by selectively depriving a subject of that particular stage and observing the results. Attempts to selectively deprive a subject of stage II sleep results in total sleep loss because it is impossible to enter other stages without going through stage II. Stage II is the stage least likely to be made up after a period of sleep deprivation, and the most likely to be increased with the use of sedative hypnotics. Stage III and IV are now collectively termed slow wave sleep or (SWS) (the only difference between the two is the absolute number of delta waves recorded on the EEG). In contrast to stage II, this stage is most constant from individual to individual and most consistently made up after a period of sleep deprivation. SWS is felt to be important for bodily repair. It is the stage during which growth hormone is secreted during normal sleep. The single most important determinant of SWS is the length of time since the last sleep episode, and as such it is not as subject to circadian factors. Rapid eye movement (REM) sleep is the most well known sleep stage. During this time the body is completely paralyzed, and in fact loses its thermoregulatory properties. This is the main time when dreaming occurs, which is thought to be important for psychological adjustment and development. Unlike SWS, REM is highly influenced by circadian periodicity.

Normally these stages cycle throughout the night in periods of twenty minutes each with relatively more SWS alternating with stage II in the earlier part of the night while REM sleep dominates during the latter part of a sleep episode. Many things can alter this sleep architecture. Drugs are an important cause of altered sleep architecture; caffeine causes a more rapid than normal cycling between stages, while alcohol suppresses REM sleep. Sedative hypnotics (with the possible exception of zolpidem) will result in greater total sleep time but almost exclusively increase stage II sleep (which may not be particularly restorative). Noise, even if it doesn't awaken one, will alter sleep cycles. Circadian placement of sleep is also very important.

Daytime sleep is typically one and one-half to two hours shorter than a nocturnal sleep period. REM and to a lesser extent stage II are the most shortened. This compounds the problems of alertness on the night shift. The night worker must contend not only with the expected circadian trough of energy and alertness, but also with sleep deprivation from poor quality daytime sleep. Many shift workers develop a near obsession with sleep.

Social/Domestic Factors

The social effects of rotating shifts on the worker and his family are also important. Society seems almost to actively subvert the shift worker. Neighbors insist on mowing their lawns while a night worker is sleeping. People who would never telephone at 2 am routinely do so at 2 pm whether one is attempting to sleep or not. Education of the worker and his family is one of the most important strategies for coping with shift work. Everyone in the family must understand that shift work is a lifestyle. Most of society realizes that sometimes one must work during important social functions but they are not so forgiving if one is "only sleeping." One of the most important things any shift worker can do is to acknowledge the negative impact of shift work, and attribute more importance to circadian principles. Other industries have demonstrated greater productivity and increased job satisfaction by applying circadian principles to scheduling.

Scheduling Strategies

How best to schedule a department is one of the most important issues for adaptation to shift work. Unfortunately there is no one best schedule. Many factors must be considered including the census and acuity of the department, individual group member's preferences, group size, part time help, etc. Two of the biggest issues are number of night shifts in a row and shift length.

Night Shift Scheduling

There are two diametrically opposite approaches to scheduling.4 From a circadian perspective, the gold standard is never to rotate shifts. A group lucky enough to find someone who will work permanent nights should work hard to retain him or her and make sure they are appropriately compensated. Without a permanent night worker the best shift rotation, from a circadian perspective, is to have group members work a long string of nights, 4 to 6 weeks. The idea is that each person can group their nights for the year together and only has to shift their circadian rhythms twice, once onto nights and once back again. Everyone in the group will work hard for that one period, but have 10 to 11 months of the year when they will only work an occasional night, on the night person's shift off. It is important for the night person to stay up even on their nights off so as not to lose their hard won adaptation.

The other strategy often used in Europe is to work as few a number of nights in a row as possible, ideally one. The idea is to never reset your circadian rhythms but to maintain a constant diurnal orientation. Which strategy a group uses depends on whether one finds circadian disharmony, that jet lagged feeling as one adjusts to a stretch of nights and then back again to days, or inappropriate phasing, the realization that one will never adapt to nights and on every night shift one is used to being asleep and is least alert. Working 4 to 7 night shifts in a row is universally condemned. One suffers from inappropriate phasing during each night shift and then just when the body starts to adapt to nights, he switches back again, experiencing the worst of both systems.

The major benefit of working many night shifts in a row is that once adjusted to nights one will be alert, well rested, and provide optimal patient care, rather than struggling to stay awake for every single night shift. With the use of bright lights and possibly melatonin, rapidly adjusting circadian rhythms has become possible. Many individuals oppose this strategy for social reasons. They fear never seeing their families during night shifts. This is a key place where education and recognition of the tolls of traditional shift work come into play. This strategy is based on the assumption that one will work a certain number of nights each year. While one may not see as much of their family during their time on nights they can spend proportionately more time during the remaining 10 to 11 months and be well rested and fun to be around.

Shift Length

Another major decision for any group is how many hours in a row to work. Traditionally most groups have worked two primary shifts of 12 hours each, with additional double coverage shifts of varying lengths as needed. There is a clear trend to shortening the primary shift. Patient care is improved with a better rested, more alert physician. There is also the ability to enjoy recreational pursuits even on workdays with eight-hour shifts. Certainly circadian principles are much more easily applied with eight-hour shifts. If a group adopts a system of many nights in a row, shorter shifts are an advantage. One universally agreed principle is to have a clockwise shift rotation from days to evenings to nights and after a break, back to days. This takes advantage of the 25-hour endogenous cycle. Most emergency physicians surveyed preferred 8-hour shifts.5

The major advantage to 12-hour shifts, is having one-third more days off completely free of hospital responsibilities. Those with a long commute are likely to favor longer shifts as well as those lucky enough to reliably get 2 to 3 hours sleep on each night shift. As physicians age or as patient census and acuity increase, most find shorter shifts more appealing. Patient care is probably improved with shorter shifts.

Other scheduling strategies for groups to consider are to change shifts at different times. Some groups work from 2 am to 10 am, 10 am to 6 pm, and 6 pm to 2 am. With this system each person gets some sleep during the normal sleep time. Many groups also vary shift length, typically making the night shift shorter, ie, 10-hour day shifts, 8-hour evening shifts, and 6-hour night shifts. It is also possible to shorten the night shift by overlapping shifts, ie, 7 am to 3 pm, 3 pm to 12 pm, 1 am to 7 am with an overlap shift from 6 pm to 2 am. The department will have single coverage for one hour from 12 pm to 1 am but when the night person arrives there will be double-coverage again until the bulk of patients has been seen.

Shift Differentials

Most other industries pay a differential for night work. One area where our specialty has been remiss is not aggressively pushing for additional reimbursement for work done at night. Groups of all sizes are beginning to reward night shifts in different ways, particularly monetarily. It is well established that working night shifts becomes more difficult as one ages and increases the potential for more errors.6 Older members who don't tolerate nights well often gladly "pay" extra to those younger members who are more tolerant of nights and less secure financially. It is relatively easy to devise a reimbursement system whereby the night shift pays relatively more and other shifts proportionately less. In other industries this helps retain valuable workers who would otherwise prematurely retire as night shifts become increasingly burdensome. Some groups reward night shifts in other ways such as fewer total shifts and/or fewer weekend and holiday shifts. Non-monetary incentives for working nights should also be considered such as decreased administrative duties.

Individual Strategies

There are also many individual strategies to help adapt to shift work. One of the most important is education of family and friends. Many studies have shown that those who are most successful at shift work don't try to live in two different worlds. Friends and family should be educated about the importance of uninterrupted daytime sleep. An answering machine is a must for all shift workers. A dark, cool, quiet sleeping place increases both total sleep time and sleep quality. Black out curtains are a worthwhile investment for all who must sleep during the day.

There are also individual sleeping strategies that can be used. A split sleep period is a technique where one sleeps for 3 to 4 hours immediately before and 3 to 4 hours immediately after a night shift. The rationale is that at least part of each sleep episode is during the circadian period when sleep is expected. Anchor sleep is a technique used following one's shift during a series of nights. It is basically a compromise to switching to a diurnal orientation. One would stay up until 3 am or 4 am and then sleep until 10 am or 11 am. That way one gets some time to socialize but doesn't completely lose a nocturnal orientation. Naps are problematic. Regularly scheduled naps can be effective and some industries with multiple workers on night shifts include them in their shift design. In general, however, random unscheduled naps serve to hinder adaptation (making it harder to get proper sleep during the planned sleep period) and do not increase alertness or improve mood.

Pharmacology must also be considered. Sedative hypnotics should not be routinely used by night workers. They are very addictive and, while they do increase total sleep time during the day, they do not hasten resetting of rhythms to night shifts or improve alertness during the night. Shift workers nearly universally use caffeine. It can increase alertness but should not be used within 4 hours of a planned sleep period. Alcohol induces sleep but the sleep is markedly distorted with little REM sleep, which is already diminished during daytime sleep. One pharmacological agent which holds promise is melatonin. This agent is a hormone secreted nightly by the pineal gland in response to darkness. Melatonin is a sedative but more importantly has been shown to hasten resetting of circadian rhythms. Several studies of jet lag have shown significant improvement with melatonin. Studies are currently underway on the use of melatonin for shift workers. Bright light of greater than 3,000 lux can also hasten resetting of circadian rhythms.7 Bright lights during the nights will increase alertness on the night shift and rapidly convert circadian rhythms. Bright light in the early morning (5 am to 7 am) can hasten adaptation back to days by phase advancing one's rhythms and allowing earlier sleep. Exercise is a useful strategy to adapt to shift work. Not only does it improve general mood but also promotes alertness on night shift (if not too strenuous). It has been shown to increase circadian adaptation also. Aerobic exercise immediately after awakening, no matter which shift one is working, is most effective.


Shift work is a necessary fact of life for emergency physicians. Emergency physicians must attribute more importance to well being and acknowledge adverse effects of shift work. This includes making rational schedules from a circadian perspective. Individual strategies should also be employed, particularly good sleep hygiene and decreasing potential interruptions.

Revised by the Wellness Section under the direction of
Harold A. Thomas, Jr, MD, FACEP
Wellness Section Steering Committee Chair
August 2003


  1. Hall KN, Wakeman MA. Residency -trained emergency physicians: their demographics, practice evolution, and attrition from emergency medicine. J Emerg Med. 1999;17:7-15.

  2. Boggild H, Knutsson A. Shift work, risk factors and cardiovascular disease. Scand J Work Environ Health. 1999;25:85-99.

  3. Whitehead DC, Thomas H, Slapper DR. A rational approach to shift work in emergency medicine. Ann Emerg Med. 1992;21:1250-1258.

  4. Monk TH. What can the Chronobiologist do to help the Shift Worker? J Biol Rhythms 2000 15:86-94.

  5. Thomas H Jr, Schwartz E, Whitehead DC. Eight- versus 12-hour shifts: Implications for emergency physicians. Ann Emerg Med. 1994;23:1096-1100.

  6. Harma, MI, Ilmarinen, JE. Towards the 24-hour society--new approaches for aging shift workers? Scand J Work Environ Health. 1999;25:610-615.

  7. Burgess HJ, Sharkey KM, Eastman CI. Bright light, dark and melatonin can promote circadian adaptation in night shift workers. Sleep Med Rev. 2002;6:407-420.

September 2003

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