001 EVOLUTION AND FUNCTION OF SLEEP

J. Horne.Sleep Research Centre, Loughborough University, Leicestershire, UK

Human sleep has much in common with that of other mammals: occurring regularly each day, influenced by a circadian rhythm; typical body posture, specific place to sleep, other behaviours and physical activity cease, eyes close, and generalised reduction in sensory awareness. The organ showing the clearest changes during sleep compared with relaxed wakefulness, is the brain; obvious in the electroencephalogram (EEG). Focusing on the brain in this way is appropriate in other respects, as not only does it contain numerous control mechanisms for sleep, but of all the body’s organs, it is the brain and behaviour (especially the cerebral cortex) for which sleep seems to be the most vital in terms of actual recovery. This is especially so for mammals with advanced cortical development, culminating in humans. For small mammals (eg rodents) which also happen to have simpler cortices, sleep has a major benefit in providing the only opportunity for physical rest, avoiding needless physical energy expenditure, and confining the animal to the thermal insulation of a nest. Here, sleep conserves much heat energy and is their “great immobiliser”. In contrast we can relax adequately during wakefulness, with only a nominal further energy saving (5–10%) to be gained by sleeping. Thus sleep serves a variety of functions in mammals, altering as the evolutionary scale is ascended, depending on body size, need to conserve energy, cerebral development, amount of relaxed wakefulness, type of diet, and safety when sleeping. Moreover, our sleep alters as a night’s sleep progresses, initially serving important purposes, changing to those of less benefit, and eventually to a sleep that is superfluous, luxurious and just pleasant to take. There is little evidence that apart from our cortex any other organ undergoes any heightened degree of change/repair during sleep. The …