When there is an important decision that we have to make we are often told to sleep over it and then decide the next day. And in fact often it is much easier to make the decision after having slept over it. From a brain science perspective there seems to be more to this idea than we might think.
Sid Kouider from Laboratoire de Sciences Cognitives et Psycholinguistique in Paris and his colleagues found that human brains make decisions during sleep. Participants in their study had to make simple decisions: one group had to decide whether spoken words were animals or objects, another group had to decide whether spoken words were real words or pseudo-words, and then they had to press a button either with their left or with their right hand. During the experiment they were allowed to nod off and their brain activity was recorded. After falling asleep of course participants stopped pushing the buttons. However, the related brain activity continued. So the brain was still responding and preparing the motor response, only that this response was not executed as it had been while participants had still been awake. However, after waking up, participants did not remember having heard the words while they had been asleep. Thus, our brain perceives what is going on around it while we are sleeping and prepares decisions, even if we are not conscious about it.
The original study was published in the journal Current Biology. The authors also describe their study on The Conversation.
What this study once more clearly shows is that our brain is far from being shut down while we are sleeping. We reported earlier that sleep is important for memory consolidation and thus for learning. We also know that our brain is very active even during meditation, so when we are not thinking.
Thus, while sleeping over a decision your brain is likely to prepare this very decision. Therefore is seems to be good advice to do this!