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[Sleep: current data in 1990].

Relations between serotonin and sleep have been re-evaluated in the waking and sleep periods. During sleep serotonin-containing neurons cease to function, as do all monoaminergic neurons. During waking one of the roles played by serotonin is to stimulate the synthesis of hypnogenic peptides. At the same time, serotonin inhibits paradoxical sleep as its occurrence during waking would be dangerous. In addition, under the influence of certain peptides serotonin, released by the dendrites of median raphe neurons, exerts a self-inhibition on their activity at the onset of sleep. The number of hypnogenic peptides discovered is ever increasing, raising the problem of their specificity and mode of action within the spontaneous sleep-waking cycle. Recent studies have reappraised the mechanisms of recovery after sleep deprivation. It is now believed that recovery is modulated by the intensity of stress during waking and not by the duration of waking, hence the hypnogenic role played by hormones, such as ACTH derivatives, that are involved in stress. Even more recent studies on the relationship between the mechanisms of sleep and those of central temperature are leading to a number of questions likely to modify the current hypotheses concerning sleep.