As interesting as my title was...

I have a Toshiba Satellite laptop, 4gigs of RAM, 500GB HD and an Intel Core 2 Duo T6600 processor. Now my problem is that, whenever I leave my computer for a certain amount of time it freezes when I try to come back on it. For example, let's say I leave right now. Then my computer shuts off it's screen, but it's still on. Then when I come back, let's saaay about 20-30+ minutes later...it will freeze for about 2-3+ minutes. I can't do anything other than move my mouse.

Why does this happen?