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Circadian Rhythms (OGHFA BN)

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Article Information
Category: Human Factors Human Factors
Content source: Flight Safety Foundation Flight Safety Foundation
Operator's Guide to Human Factors in Aviation
Human Performance and Limitations
Circadian Rhythms

Briefing Note


This Briefing Note (BN) describes the mechanisms underlying circadian rhythms and the effects of circadian rhythms on pilot performance. The information should help flight crews develop strategies to cope with the effects of flight operations on circadian rhythms. This BN describes:

  • Data on the relationship between circadian rhythms and performance
  • The biological clock that controls the circadian variations in biological and behavioral functions
  • The practical implication of circadian rhythms for night operations and transmeridian (across time zones) flights.


Flight operations that involve irregular work hours, night flights, early starts or transmeridian flights force pilots to deviate from their normal work/sleep schedule and disrupt their biological rhythms. Many of our biological and behavioral functions experience variations throughout the day, including: sleep, body temperature, alertness levels and mental and physical performances. Many of these functions vary systematically in a cycle of about 24 hours and are called "circadian rhythms" (from the Latin words "circa" which means "about" and "dies" which means "a day"). These circadian variations are governed by a biological clock located in the brain. Crewmembers who work abnormal schedules often experience “shift-lag syndrome,” which is characterized by such symptoms as feelings of fatigue, sleepiness, insomnia, disorientation, digestive trouble, irritability, reduced mental agility and reduced performance efficiency. Similar symptoms labeled “jet-lag syndrome” are often experienced by crew members after transmeridian flights.

The mechanism underlying circadian rhythms is called the “biological clock” or the “circadian clock.” Research has shown that the biological clock is located in the suprachiasmatic nucleus[1] of the hypothalamus (a gland). The biological clock is probably the result of human evolutionary adaptation to the solar day.

Laboratory studies have shown that, in the absence of any time cues (i.e., no sunlight or social time cues), the biological clock for most humans operates on a cycle of about 25 hours. Under ordinary circumstances, however, the biological clock is reset by about one hour each day such that the biological clock is synchronized with the 24-hour solar day. The cues that serve to reset the biological clock are called “zeitgebers,” a German word that means “time givers.” Evidence supports morning sunlight as the most important zeitgeber. Other cues in the social environment that serve as zeitgebers have not been identified with any amount of certainty. However, cues that may serve as zeitgebers include work/sleep schedule, eating schedule, social activities and, in the absence of other cues, subtle environmental factors such as building vibration and traffic noise.

Although the biological clock can routinely be reset by about one hour each day, it cannot easily and quickly be reset by the large time quantities that are needed following significant changes in work/sleep schedule or a transmeridian flight. The slow adaptation of the circadian clock contributes to problems in conducting night operations and transmeridian flights.


Many laboratory studies have demonstrated circadian variations in biological functions such as body temperature, cell division and hormone secretion. Also, both laboratory studies and field studies have demonstrated variations related to circadian rhythms in behavioral functions such as alertness, reaction time, short-term memory, long-term memory, search tasks, vigilance and sleep. The circadian variation throughout a normal solar day is not the same for all biological and behavioral functions. There are, however, general trends in certain bodily functions/parameters likely related to circadian relations.

The body temperatures of individuals adapted to local time and to a normal work/sleep cycle (i.e., sleep at night) vary systematically with circadian rhythms. Body temperature is lowest during the early morning hours from about 2 am to 6 am and starts to rise from this low point at about the normal waking hour. Thereafter, body temperature tends to rise until late afternoon or early evening, at which point it starts a decline that continues until it reaches its low point in the early morning hours. The circadian variation in body temperature is virtually the same for active and non-active individuals. It has been suggested that body temperature is an indicator of the body’s readiness to perform work.

The results of research support several conclusions about circadian rhythms that are useful in maximizing pilot performance. Circadian variations in work efficiency are not the same for all tasks. Also, under a normal work/sleep schedule and complete adaptation to the local solar day, performance efficiency does not remain the same throughout the day. For many tasks, performance efficiency tends to increase from the normal wake-up time in the morning to a peak in the early or late afternoon. Performance efficiency on some tasks shows a temporary decline following lunch time, even if a meal is not eaten. It is important to point out that work efficiency in these studies was tested periodically (and briefly) throughout the day (about 8 am through 9 pm), so fatigue was not a factor affecting performance. Performance efficiency tends to decline to a low point in the early morning hours ( 2-6 am). The important implication of this research is that circadian rhythms influence performance efficiency even when the circadian variations are in synchrony with the solar day and the normal work/sleep schedule.

The effects of circadian rhythms on safety are difficult to assess because they are virtually always confounded with other contributory factors. However, the following findings suggest that the effects of circadian rhythms are, in part, responsible for:

  • The number of motor vehicle accidents on roadways peaks between 2 am and 6 am and again around 3 pm. These are the times of maximum sleepiness due to circadian rhythms
  • Risk of injury is 30 percent higher during night shifts than during day shifts, and the difference increases over successive night shifts until the difference reaches a high of 39 percent increased risk of injury on the fourth night.

Research also has demonstrated that a host of problems occur when circadian rhythms are not in synchrony with the work/sleep schedule imposed by a person’s job. Such asynchrony can result from a change in work schedule, transmeridian flight, or a combination of the two. Such asynchrony is important for two reasons. First, the job may require an individual to perform work at a phase during the circadian cycle when performance efficiency is low. Second, disrupting the normal work/sleep schedule decreases the amount and quality of sleep, which leads to fatigue .

Night Operations

Night operations create a host of problems for flight crews. The primary problem is having to work efficiently and safely at a point in time when the work requirements are not in synchrony with circadian rhythms. Under worst-case conditions, crew members must perform demanding tasks during the early morning hours (2 am to 6 am) when their biological functions and performance efficiency are at their lowest level. This problem cannot be quickly solved by adaptation of the biological clock. Complete adjustment to night work requires at least 21 night shifts in a row with no days off. Adjustment of the biological clock does not even commence until about 10 days after a shift change. In fact, it has been argued that crewmembers never fully adapt to night operations because

(a) they do not stay on the night shift long enough to adapt fully and
(b) they revert to a regular routine during their days off, thereby stopping or reversing the adaptation process.

In addition, the light-dark cycle works against adaptation to night operations. The morning sunlight experienced during the drive home from work prevents adaptation by resetting the biological clock back to the normal solar day.

As stated earlier, prolonged asynchrony between circadian rhythms and work requirements causes crew members to experience shift-lag syndrome, which is characterized by feelings of fatigue, sleepiness, insomnia, disorientation, digestive trouble, irritability, reduced mental agility and reduced performance efficiency

Sleep difficulties are a major problem for crew members who participate in night operations. Both the duration and quality of sleep are affected. Daytime sleep following night operations is generally of poor quality due to shorter durations than normal night sleep and because it is more susceptible to interruption, which results in fewer and shorter periods of deep sleep. After a period of night work and daytime sleep, a sleep deficit can accumulate that results in cumulative fatigue. This cumulative fatigue can further exacerbate the difficulty of maintaining efficient and safe performance during night operations.

There is some evidence that the effect of night work is more severe among older workers, and shift workers are more tired when driving to and from work than non-shift workers.

For the reasons discussed above, the adaptation to night work is never complete. More complete adaptation can be achieved for permanent night work than rotating shift work or irregular work hours, but the requirements of air operations seldom enable flight crew members to work the same shift for more than a few days.

Jet lag

Symptoms of jet lag include feelings of fatigue and inertia, difficulties in concentrating and sleeping, gastrointestinal problems and a general malaise. The syndrome is distinct from so-called “travel fatigue,” which is the tiredness experienced after a long and often stressful journey. Travel fatigue occurs for both transmeridian flights (east/west across time zones) and translatitude flights (north/south with little or no time change). With travel fatigue, there may also be residual stiffness due to remaining in a cramped posture for a long time. The effects of jet-lag syndrome on the individual's mental performance may be subliminal and go unnoticed while other symptoms may be more obvious during the period of adjustment to the new time zone.

Under normal conditions, the biological clock is in phase with the environmental synchronizers. The period of least efficiency coincides with the nocturnal period, and the period of optimal efficiency coincides with the diurnal period. At the end of a transmeridian flight and for a period thereafter, the circadian system and environmental synchronizers are out of phase.

Table 1 illustrates the mismatch between “body clock time” and local clock time following a transmeridian flight that covers eight time zones in an eastward direction.

Table 1. Mismatch between local times and “body clock time” immediately after an 8-hour time-zone transition eastwards.
Origin Local Time Normal Desire Destination local time Requirement
00.00 Sleep 08.00 Waking
08.00 Begin to wake 16.00 Peak activity
16.00 Peak activity 24.00 Retiring

As stated earlier, the biological clock does not immediately adjust to new time zones. The amount of time required for the biological clock to adjust to a new time zone depends on the individual, the direction of flight, the number of time zones crossed and the individual’s exposure to environmental cues.

The direction of the time zone change has been shown to have a substantial affect on adaptation time. Adaptation after eastbound flights is about 50 percent slower than after westbound flights. For eastbound flights, about 1.5 days of recovery time is required for each time zone change compared to about one day of recovery time for each time zone change in westbound flights. The difference in recovery time is due, in part, to the fact that the free-running cycle of the biological clock is longer than 24 hours. The difference is largely a function of differences in adapting to a new sleep schedule. Indeed, the adjustment after eastbound flight requires a crewmember to go to sleep and get up earlier while adjustment after westbound flight requires a crewmember to go to sleep and get up at later hours.

In addition to this differing rate of adaptation due to direction of travel, psycho-physiological functions adjust at various rates depending on the individual. It is also relatively common for travelers to adapt in the wrong direction, such as delaying 16 hours instead of advancing 8 hours[2].

Defenses Against Circadian Rhythm Disruptions

A number of strategies can be used to counteract the effects of transmeridian and translongitudinal flight. To counteract disruptions to your circadian rhythms:

  • Know your normal body clock times for sleeping and eating by using the Body Clock Questionnaire (BCQ)
  • Determine how you are adjusting to local time during layovers by using the Layover Adjustment Questionnaire (LAAQ)
  • Based on the BCQ and LAAQ, attempt to modify your sleeping and eating schedules to adjust for maximum alertness. Try to only eat meals and drink coffee or tea at times when your sleep will not be adversely affected
  • Use good nap management before and during flight
  • Coordinate rest and meal periods with other crewmembers
  • Exercise at appropriate times
  • Expose yourself to sunlight at appropriate times

Summary of Key Points

  • Many human biological and behavioral functions vary regularly and systematically over a period of about 24 hours. These variations are called circadian rhythms
  • Circadian rhythms persist even in the absence of all environmental and social time cues
  • Circadian rhythms are internally generated by a self-sustaining or autonomous biological clock located in the hypothalamus
  • In the absence of all time cues, the biological clock has a natural cycle of about 25 hours. With normal time cues, however, the biological clock is reset each day such that it is in synchrony with the solar day.
  • Changes in work shifts and transmeridian flight result in asynchrony between a crewmember’s circadian rhythms and both work requirements and environmental time cues
  • This lack of synchrony results in shift-lag syndrome (due to changes in work schedule) and jet-lag syndrome (due to transmeridian flights)
  • The biological clock and the associated circadian variations adapt slowly following changes in the work schedule and following transmeridian flights
  • Adaptation after eastbound travel is about 50 percent slower than after westbound flight -- adaptation time following eastbound travel is about 1.5 days for each time zone change whereas adaptation time following westbound travel is about one day for each time zone change
  • The adaptation rate is not the same for all of the circadian biological and behavioral variations. The resultant disharmony among these functions contributes to jet-lag syndrome.


  1. ^ The suprachiasmatic nucleus or nuclei, abbreviated SCN, is a tiny region on the brain's midline, situated directly above the optic chiasm. It is responsible for controlling circadian rhythms. The neuronal and hormonal activities it generates regulate many different body functions in a 24-hour cycle, using around 20,000 neurons.
  2. ^ Gundel, A., and H.M. Wegmann - Transition between advance and delay responses to eastbound transmeridian flights. Chronobiol. Int. 6: 147-156, 1989.

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