If you're tired, you probably need to sleep so lie down and close your eyes. When you wake up, you'll feel much better.
Everyone needs sleep and most of us get some at least once a day. Maybe that's why sleep is used in so many expressions. If you “sleep on it” you'll wait to make a decision until tomorrow. If you “sleep over,” you stay at someone's place for the night. A person who “lets sleeping dogs lie” knows when not to stir up trouble. If you slip into eternal sleep, you won't be needing any more naps.