ABSTRACT
There are three main types of theory of active sleep (AS) which currently dominate the international research scene. The first type suggests that AS exists in order to exercise the brain at intervals during sleep. The second type of theory suggests that AS permits some special type of recovery which cannot take place as effectively during wakefulness or quiet sleep (QS). The third type of theory emphasises the biological advantages of the brief awakening which occurs at the end of an AS episode in many species. It is generally believed that AS is a newcomer on the sleep scene, and that QS, or some version of it, has been with us since time immemorial. The prior arrival of AS during infant development does not prove that AS evolved before QS but it does make it very plausible. The gradual decrease in the AS proportion of total sleep continues into adulthood, where it is almost always less than 25 per cent.
