Kennedy, Gerard, Howard, Mark and Pierce, Rob (2001) Temporal Nature of Road and Workplace Accidents and Effects of Naps and Sleep Inertia. Project Report. UNSPECIFIED. (Submitted)

Abstract

Many biochemical, physiological, and behavioural processes in humans and other animals show oscillations, or cycles, that are close to 24 hours in periodicity. These oscillations are known as circadian rhythms. During different times of the day dramatic differences can be seen in physiological and psychological processes. The most obvious difference is that individuals are at rest for part of the day and are extremely active at other times (Monk et al., 1985). The peak activity associated with the circadian rhythm of most physiological functions occurs in the afternoon, while the circadian trough or nadir normally occurs in the early hours of the morning (Harma & Ilmairnen, 1999), thus following a biphasic pattern. Circadian rhythms are controlled by a master biological clock located in the suprachiasmatic nuclei of the hypothalamus (Rajaratnam & Arendt, 2001). The main internal marker of biological- clock activity is the rhythm of melatonin and cortisol (Rajaratnam & Arendt, 2001). Melatonin is synthesised and secreted at night, thereby acting as a signal for the length of day and night. External influences that help to synchronise circadian rhythms include environmental influences such as the daily light-dark cycle, noise, external temperature, meals and work or school schedules (Wever, 1981). The most pervasive synchronising agent is the light-dark cycle (Sack & Lewy, 1997), which entrains the sleep-wake cycle to 24 hours (Akerstedt, Hume, Minors & Waterhouse, 1993). When circadian rhythms are entrained to 24 hours by environmental and social factors, there is a mutual coupling between the different biological rhythms of the human body (Sack & Lewy, 1997). If circadian rhythms become desynchronised, the sleep-wake cycle can act as a phase controller thereby playing an important role in the maintenance of circadian rhythmicity (Aschoff, 1981). When the sleep-wake cycle and the light-dark cycle are synchronised an individual is able to perform at a maximum capacity during the day, and experience optimum sleep quality and duration during the night (Efinger & Nelson, 1995). Attempts to sleep at inappropriate times during the circadian cycle, such as during the declining phase of melatonin, can result in shorter sleep episodes and more awakenings. Such attempts are frequent in night shift workers. Disruption to the circadian rhythm can upset physiological factors, such as motor activity, sleep/wake cycle, blood pressure and work performance, and therefore should be an important consideration in the study of shift worker and driver fatigue (Lal & Craig, 2001).

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