Shiftwork and Circadian Variations of Vigilance and Performance

Man is a day-light creature, having associated his own state of wakefulness and activity with the day-light period and, consequently, his rest and sleep state with the night-dark period, with a regular rhythm over the 24-hour span. This conditioning derives from the evolutionary adaptation of the human species to the physical environment and from the social habits of each individual. In relation to this periodic alternation, all the body functions (metabolic, respiratory, digestive, cardio-circulatory, renal, nervous) show a circadian (“circa diem” = about 24 hours) rhythm, with higher functional levels during the “ergotropic” phase (light-wake-activity) and lower levels during the “trophotropic” phase (dark-sleep-rest). This circadian rhythm is sustained by an endogenous oscillator or “body clock,” localized in the brain at the level of the suprachiasmatic nuclei, and modulated by external influences of such socio-environmental “synchronizers” as light period, timing of sleep, activity pattern, and meals. Under normal conditions of the sleep/wake cycle, the internal and external factors are in constant relation, determining a clear circadian variation of the psycho-physiological state (Minors and Waterhouse, 1986).