Abstract

Sleep is often viewed as a vulnerable state that is incompatible with behaviours that nourish and propagate species. This has led to the hypothesis that sleep has survived because it fulfills some universal, but as yet unknown, vital function. I propose that sleep is best understood as a variant of dormant states seen throughout the plant and animal kingdoms and that it is itself highly adaptive because it optimizes the timing and duration of behaviour. Current evidence indicates that ecological variables are the main determinants of sleep duration and intensity across species.

Author affiliations

1. Jerome M. Siegel is at the Department of Psychiatry, School of Medicine, University of California, Los Angeles, California 90095, USA and at Neurobiology Research (151-A3), Veterans Affairs Greater Los Angeles Healthcare System, North Hills, California 91343, USA.

Correspondence to: Jerome M. Siegel¹ Email: JSiegel@ucla.edu

Published online 5 August 2009

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