There are over 20 definitions of "sleep" in several dictionaries. The first, a verb, seems most appropriate:
for example: to take the rest afforded by a suspension of voluntary bodily functions and the natural suspension, complete or partial, of consciousness; cease being awake.
Physiologically, sleep is a complex process of restoration and renewal for the body. Scientists still do not have a definitive explanation for why humans have a need for sleep. We do know that sleep is not a passive process or "switching off" of body functions; sleep is believed to be important in many physiologic processes including the processing of experiences and the consolidation of memories. It is also clear that sleep is essential, not only for humans but for almost all animals...
in other words, sleep well our brains to Rest a while after working all day. So now time for bed ... nite ..