"Sleep On It"

We've all heard the term "Sleep on it" before.  It's a term something someone says to a person when there's something wrong because sleeping usually makes a person feel better.  But the more I think about it, why is that?  If you're tense when you go to sleep, your body heats up, which is why it's difficult to sleep when it's hot around you.  Also, it's been said that sleeping when you are thinking about someone or feeling negative will cause you to dream (something I can't really confirm though, since dreams represent so many various things).  Case in point, I had a terrible day yesterday because of a school issue, and I had a tough time falling asleep, but once it happened, it happened quickly, and when I woke up, I felt refreshed and better.  I wonder why that is?  Does rest really help that much, or is it something more, such as something psychological?  As much as I want to know, I know I can't figure it out now, so I'm gonna have to sleep on it and get back to you.