Most physiological, biochemical and behavioural functions in the body vary rhythmically across the 24hr day. This circadian clock also synchronizes or entrains our daily rhythms with the prevailing light-dark cycle.
Sleep is a period of inactivity with raised thresholds to arousal by external stimuli.
However sleep is an active process. While some brain areas are inhibited others remain active. This activity is sometimes called beta activity. In Stage 1, the initial period of sleep, alpha waves gradually decrease in amplitude and become more irregular in frequency. Species differ in the type of sleep they display.