Sleep on it

Sleep on it:
  
In spoken English, if someone tells their friend to, "sleep on it", it means that they should really think carefully before making a decision.  This is used when someone has a big and important decision in life to make.  The idea is that you should go to sleep and make a final decision the next day so you don't make an emotional decision.
 
Here is an example of how to use this:

Tom:  I'm thinking about buying a new i-pad but it's kind of expensive and I'm not really sure if I really need it or not.  Do you think I should get one?
 
Riley:  I think you should sleep on it.  If you still want one tomorrow morning then just buy it.