Circadian Rhythms

Human beings like most organisms, demonstrate daily behavioural and physiological rhythms. These daily or circadian rhythms control and regulate bodily functions. In aviation, shift work and night flights have a disruptive influence on circadian rhythms which significantly influences human performance.

What are Circadian Rhythms?

Nearly all human bodily functions show significant daily variations including arousal, psychophysical performance, food and water consumption, metabolism, body temperature, heart rate, blood pressure, hormone production, and digestive acid secretion. These variations follow a daily cycle that is a complex interplay between the bodies circadian rhythm or 24-hour cycle and homeostasis.
Homeostasis are the control mechanisms for the body’s internal environment functions, such as, temperature regulation, salinity, and acidity, keeping required levels within a fairly narrow operational range to maintain an internal equilibrium within the body (Britannica, 2005). In essence it is a physiological mechanism that maintains desirable bodily function levels; however, homeostasis is not a daily cycle in itself. Circadian rhythms are one of the primary triggers of the homeostasis process that influence the relevant changes in bodily functions.

The circadian rhythm is traditionally regarded as the biological ‘time-clock’ that regulates the body’s daily sleep-wake patterns (Edery, 1999). The term circadian originates from Latin: circa (about) and dies (day). The circadian rhythm is an internal cycle that occurs within a period of approximately 24 hours.

(Image embedded from on 17 August 2011)

Synchronisation of the circadian cycle is influenced by environmental signals, with the daily light-dark cycle being the dominant cue; other cues include diurnal temperature variation and timing of meals. While the cycle is influenced and synchronised from outside zeitgebers, or time cues, the rhythm itself, is generated from inside the human organism. A fundamental feature of the circadian cycle is that it will continue, even when isolated from time or environmental cues (Colwell, 1998). The constancy of the circadian cycle generates problems for shift workers and aircrew, as the circadian rhythm will take several days to adjust to differing, eating patterns, temperature, light, and sleep patterns.
The circadian rhythm’s most dominant attribute is its effect on the human sleep cycle; this combined with its influence on body temperature regulation, hormone production and adrenal gland output, cause the body to desire sleep. Sleep is desired and initiated at preferred times relative to the circadian rhythm of core body temperature (Monk, 1987). In general, the longest and best quality sleep episodes are initiated several hours prior to the body temperature minimum.
A human is most productive when their body temperature is at its highest and logically at its least efficient, when temperature is lowest. Human performance degradation at circadian lows is one of the major challenges for the aviation industry.

Circadian Rhythms and Human Performance

It is generally accepted that human performance declines at night, when the body and mind desire rest (Monk, 1987; CAA, 2003; Stokes & Kite, 1997).
A ‘normal sleep period’ is when the body and mind restore from the days exertions and prepare for the next period of wakefulness. The normal sleep period contains several sleep cycles of about 90 minutes, with four to five cycles in an 8-hour sleep period. Research indicates that several short periods of sleep do not have the same mental restorative properties as one long period (CAA, 2003). Consecutive nights of short or disrupted sleep have a cumulative effect on a person’s alertness level and result in a reduced cognitive performance. Indeed as little as 2 hours sleep loss can result in a measurable performance decrement (Weinberg, Jantzen, & Cheyne, 1998).
A 1995 Airbus study noted that the quality of sleep experienced by night shift workers is significantly worse than a normal night sleep period. Post night shift sleep periods are reduced in length, as the wake up occurs as the body temperature starts to rise near midday. Furthermore shift workers often have two shorter periods of sleep, one after and one before the night shift. The amount of Rapid Eye Movement (REM) sleep a person experiences during a sleep cycle increases with the length of the sleep period. Research indicates that several short periods of sleep do not have the same mental restorative properties of one long period, due to people not getting as much cognitively restoring REM sleep (CAA, 2003).

Physical effects of Circadian Disruption

The NASA Ames Fatigue Countermeasures Program has underlined the fact that sleep loss, and circadian disruption lead to significant decrements in alertness and performance (Neri, et al., 2002). A 1995 Airbus report highlights Human performance decrements include problems with

  • Vigilance
  • Alertness
  • Irritability
  • Loss of mental agility
  • Ability to multi-task
  • Decision making
  • Perceptive skills

The challenge facing flight crews is the need to maintain vigilance during long, highly-automated, and often boring night flights. Traditionally most accidents and incidents occur during the approach and landing phase of flight (FSF, 2001).

Steps to counteract the effects of Circadian Disruption

To combat reduced air crew performance the aviation industry has developed several strategies that attempt to address the problem, including:

  • Computerised Fatigue Risk Management (FRM) rostering systems that have fatigue models built in. These programs attempt to produce a flight crew roster that takes into account circadian rhythm disruption, jet lag and rest patterns.
  • Published company procedures for sleep and fatigue management, particularly for Ultra-Long Range (ULR) operations. Sound organisational policies, regarding rostering practices and fitness to fly.
  • Education of employees on stress, sleep and fatigue management. Through education crew members can improve performance by their own efforts. This includes good sleep and rest practices, physical activity and controlled use of stimulants, such as coffee.
  • Support of controlled rest or napping. Crewmembers who are allowed to take planned naps show better performance and higher physiological alertness during the last 90 minutes of flight than crewmembers who had not napped (Rosekind, et al., 1995).
  • Crew Rest facilities for ULR operations, that feature a temperature controlled, quiet, dark environment with flat beds or bunks.
1. Airbus. (1995). Coping with long range flying: Recommendations for crew rest and alertness. A1/ST-F 472 7057, 172-174.
2. Encyclopaedia Britannica, (2005). Human disease. Encyclopaedia Britannica.
3. CAA. (2003). A Review of In-flight Napping Strategies. CAA Paper 2003/8.
4. Colwell, C. S. (1998). Circadian Rhythms. Psychopharmacology: the fourth generation of progress. Lippincott-Raven.
5. Edery, I. (2000). Circadian rhythms in a nutshell. Physiology Genomics, Aug 2000; 3: 59-74.
6. Flight Safety Foundation. (2001). Approach and Landing accident Reduction (ALAR) Tool Kit (CDROM). Flight Safety Foundation. 2000/2001.
7. Monk, T.H. (1987). Subjective ratings of sleepiness - The underlying circadian mechanisms. Sleep 1987; 10(4): 343-53.
8. Neri, D.F., Oyung, R.L., Colletti, L.M., Mallis M.M., Tam, P.Y., & Dinges, D. (2002). Controlled breaks as a fatigue countermeasure on the flight deck. Aviation, Space, and Environmental Medicine 3(7):654–664.
9. Rosekind, M. R., Smith, R. M., Miller, D. L., Co, E. L., Gregory, K. B., Webbon, L. L., Gander, P. H., Lebacqz, J. V. (1995). Alertness management: strategic naps in operational settings. Journal of Sleep Research; 4/S2: pp62-66.
10. Stokes A., & Kite K. (1997). Flight Stress: Stress, fatigue and performance in aviation, Avebury Aviation, Aldershot.
11. Weinberg, H., Jantzen, J.J, & Cheyne, D. (1998). Measurement and monitoring of the effects of work schedule and jet lag on the information processing capacity of individual pilots. Transport Canada report TP 13193E.

Contributors to this page

Maria Pearson

Louise ODonnellLouise ODonnell

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License