Adverse effects of rotating schedules on the circadian rhythms of air medical crews

Abstract

Air Medical Journal 14:2 April-June 1995 Introduction Observations of the rhythmic nature of animal behavior date back to the time of Aristotle, who noted swelling of sea urchin ovaries during the full moon.1 “Sleep movements” of plants also were noted by the ancient Greeks, who attributed the phenomena to the presence or absence of sunlight. Circadian rhythm (Latin: circa meaning about, and diem meaning a day) is defined as an inborn, genetically programmed, self-sustained rhythm in behavior, physiology and metabolism, developed over evolutionary time that enables living organisms to cope with the 24hour daily rotation of the planet.’ One of the first experiments designed to test circadian rhythms in plants was done in 1729 by de Mairan, who illustrated that plant leaves open in the daytime even when kept in total darkness.2 This was the first presentation of evidence of an endogenous biological clock that functioned in the absence of zeitgebers (German for “time givers” or clues to the time of day). The term “free running” has been used to describe a rhythm in behavior pattern, or resultant metabolic pattern, that continues in the absence of any exogenous clues.1 The concept of freerunning biological clocks was discovered in 1832 by de Candolle, who placed plants in constant bright light.2 After a few days, the leaves opened and closed on a 22-hour cycle. This display indicated that the plant had its own independent day length. Circadian rhythms have endogenous and exogenous components; the former is dependent on a clocklike mechanism, while the latter is driven by external time clues (zeitgebers).z The endogenous clock appears to be responsible for the free-running day length, and light/dark cycles have been shown to be important in adjusting the human circadian clock. Many species of invertebrates demonstrate that circadian clocks have evolved mechanisms by which they frequently are reset to keep pace with changes in the local environment, most notably by being sensitive to the intensity of light.3 Controlled experiments have shown that humans have a 25.1-hour circadian clock that is constant and predictable but entrained to the 24-hour day by environmental clues.294 Volunteers were studied by Weitzman and Czeisler in periods ranging from two weeks to six months, during which they were isolated from any zeitgebers. Subjects were permitted to eat and sleep at times of their own choosing, and after one week in the clue-free environment, they lagged behind their usual day/night schedules by almost eight hours. This implies that human internal clocks are reset on a daily basis.2 Forced changes in circadian rhythm produce sleep disruption and deprivation.2,5-7 The circadian system influences performance through its effects on metabolic functions and on quality of sleep. Its rhythmicity and consistency is crucial to the maintenance of normal levels of body function and performance. The most obvious change occurs in sleep patterns, where sleep becomes shortened and disrupted by the displacement of the circadian rhythm. Pilots undergoing a shift change may find themselves awake in the