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Circadian rhythm

The circadian rhythm is the name given to the ‘internal body clock’ that regulates the approximately 24-hour cycle of biological processes in animals and plants. The body’s physiological processes are not constant throughout the day, but are rhythmic and cyclical. There are circadian patterns for cognitive and psychomotor performance, physiological activities (e.g. digestion, immune function, temperature) (see Figure 8.3), alertness and mood. Even birth and death have circadian patterns that peak during the night (Kryger et al., 1994).

Typical core body temperature circadian cycle

Figure 8.3 Typical core body temperature circadian cycle

Laboratory studies depriving volunteers of any time cues have shown that the circadian cycle actually runs slightly longer than 24 hours if allowed to ‘free run’ in the absence of external cues such as daylight (Czeisler et al., 1982). The circadian cycle is biphasic: our levels of alertness peak between 12:00 and 21:00 (usually around 16:00) and fall to a minimum between 03:00 and 06:00 hours. Evidence from traffic accidents and occupational accidents shows that a peak tends to occur in the early hours of the morning (between 1am and 6am), when performance is at its lowest. Further, particularly for older drivers, there is a secondary accident peak between 1pm and 5pm (Langlois et al., 1985).

There are individual differences in circadian rhythm. Questionnaires, such as the Morningness-Eveningness questionnaire, have been widely used to quantify individual propensity to be active and alert in the morning or in the evening (Horne and ostberg, 1976). one in ten people is a morning person (‘lark’) who naturally likes to wake up early in the morning, and about two in ten are ‘owls’, who enjoy staying up long past midnight (Smolensky and Lamberg, 2000). The rest of us are somewhere between these two extremes. Thus, someone who is an owl may be more suited to night work than someone who is a lark.

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