what is sleep?please explain in lay mans words.

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the other day i was sitting in a clinic of my doctor waiting for my turn.i had to wait five hours for it,thats a different thing.there i tried to not to fall asleep.i failed misarably.i found myself snoring.i woke up when my mouth became dry.so i want to know what is sleep?why do we sleep in odd places, in odd postures,at odd hours?why?my efforts to resist fail.to stay awake i walk around.bot i can not walk around in small cubicle of six by six,filled with other patients.i cannot sleep in the night.i get up, roam around the rooms, straitening this, checking that.if sit in place for more than ten minutes without moving i start nodding my head.i want to know the reason for this strange happenings