The ubiquitous periodic alternation of sleep and waking periods of variable duration (and by analogy of rest and activity periods) that is present in all animal species including, according to recent investigations, the fruit fly.
See Active sleep, Behavioral state concept, Mesencephalic reticular activating system, Metencephalon, NREM sleep, Pineal gland, REM sleep, Suorachiasmatic nucleus (SCN)