Ontogenetic considerations

A second important consideration is that sleep is not static across the life-span. For example, in mammals sleep amounts, brain activity, and sleep regulation undergo dramatic transformations during development [31]. The most prominent changes are in the amounts and types of sleep, with overall sleep amounts (and REM sleep in particular) being much higher in early life than at any other point in the lifespan [31]. The regulation of sleep is also quite different in developing mammals. Circadian rhythms in sleep and wake are not observed at birth, even though the states of REM and non-REM (NREM) sleep are present in many species [31]. The classic homeostatic increases in EEG slow-wave activity and REM sleep amounts following sleep deprivation are also absent in neonatal/juvenile rats [32,33] and appear quite abruptly coincident with changes in sleep-deprivation-induced neurotrophins [34]. On the other hand, neonatal sleep time is finely regulated [32], which suggests (see below) that it must serve some purpose for the developing animal. Sleep in developing non-mammalian vertebrates and invertebrates has only been minimally investigated, but similar changes in sleep amounts across the life-span may occur [32,33]. A unifying theory of sleep function should therefore account for these dramatic changes in sleep expression. It should also address whether the presumed function is different in developing animals or preserved in some fashion across the life-span.

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