These words all relate to entering into or being in the state called sleep. The most formal term in the group is retire . It can mean simply to go to bed: We retired early that night because the day’s activities had been so strenuous. It can and often does suggest a withdrawal to a private place where one can be alone, read, write, etc., before actually going to bed: We went to see Wilson after dinner, but his housekeeper informed us that he had retired for the evening. Fall asleep and go to sleep are the most common terms in the group. The first stresses the natural, passive suspension of consciousness that occurs when one ceases being awake: I fell asleep as soon as my head hit the pillow. The second can mean the same thing: to go to sleep quickly. But it can also suggest the deliberate action involved in going to bed before falling asleep : Let’s go to sleep now and clean up this mess in the morning.

Turn in is a colloquial equivalent for go to sleep in the sense which implies the action of going to bed: We turned in at ten and slept for twelve hours straight. It can also suggest the same kind of withdrawing that retire does: I turned in right after dinner so as to get back to the detective story I’d started the night before.

Hit the hay and hit the sack are both slang expressions meaning to go to bed. Hit the snack was widely used in the armed services during World War II, but is now known in civilian as well as in military life. Flake out , believed to have stemmed from the nautical term to flake out a rope on the deck, is slang for go to bed, but it can mean fall asleep and even can suggest sleeping for a long time or at least for as long as one wishes: It’s going to be great to flake out tonight after a whole day on mess duty. Flak out has also developed the sense of passing out as a result of having had too much to drink: Joe flaked out early in the piece, and didn’t come good until the party was almost over.

The slang expression grab some shut-eye means to sleep. It can be used to refer to an ordinary night’s rest or to even a longer than normal period. But it often indicates a short sleep or nap: I’m going to grab some shut-eye before the party begins.