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sleep on something
1. Lit. to recline on something and slumber. I like to sleep on a firm bed. Mary had to sleep on the floor because her sister was visiting.
2. Fig. to postpone a decision until one has slept through the night. (As if one were going to think through the decision while sleeping.) It sounds like a good idea, but I'd like to sleep on it before giving you my response. You go home and sleep on it and give me your answer in the morning.
To think about something overnight before deciding: If you can't make a decision right now, sleep on it, and give me your answer in the morning.