Figure 1

Schematic representation of basic processes underlying sleep regulation: a homeostatic process mediating the rise in 'sleep pressure' during waking and its dissipation during sleep; a circadian process (clock-like mechanism, basically independent of prior sleep and waking) defining the alternation of periods with high and low sleep propensity; an ultradian component occurring within sleep reflected by the alternation of nonREM and REM sleep. The progressive decline of nonREM sleep intensity is represented both in the top and bottom diagram (decline of ultradian amplitude). The increase in the duration of successive REM sleep episodes is indicated. W: waking; S: sleep; N: nonREM sleep; R: REM sleep. (Adapted from ref. 26)

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published 2000