From Live Science:
We're often told, "You should sleep on it" before you make an important decision. Why is that? How does "sleeping on it" help your decision-making process?
Conventional wisdom suggests that by "sleeping on it," we clear our minds and relieve ourselves of the immediacy (and accompanying stress) of making a decision. Sleep also helps organize our memories, process the information of the day, and solve problems. Such wisdom also suggests that conscious deliberation helps decision making in general. But new research (Dijksterhuis et al., 2009) suggests something else might also be at work — our unconscious.