Sleep is essential for restoring the normal balance between the different parts of the central nervous system. During sleep, the body's physical functions are rested and some renewal takes place; sympathetic nervous activity decreases and the muscular tone becomes almost nil; the arterial blood pressure falls, the pulse rate decreases, the blood vessels in the skin dilate and the overall basal metabolic rate of the body falls by up to 20%. On average, most humans need physiologically about 8 h of sleep per night; however, in modern society most adults report an average of 7-7.5 h sleep per night, with 75% reporting daytime sleepiness (Rosekind et al., 1994).
Sleep loss can be acute or cumulative. In an acute situation, sleep loss can occur either totally or as a partial loss. It can accumulate over time into what is referred to as 'sleep debt'. As little as 2 h of sleep loss can result in impairment of performance and levels of alertness. Sleep loss leads to increased reaction time, reduced vigilance, cognitive slowing, memory problems, time-on-task decrements and optimum response decrements. It has also been shown that performance variability increases with sleep loss.
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