Also, how are you trying to wake it up? Some motherboards don't transmit power to keyboards/mice when in sleep, so you consequently can't wake them up with it. Even then, pushing (not holding) the power button should wake it up.
I use the mouse or the key board. If it is asleep for a short while I push the power button, and it wakes up, but if it's been a couple hours then it wont wake up have to hold the button power down come back on, then it say's restarting windows 7.
Make sure that in the power settings where you set the time to sleep you have to go into the advance settings and make sure you don't have the hard drives shutting down after so long. You have to have that setting at never or the hard drives will shut down so it's actually off - not sleeping. And in that state boiler1990 is correct that just pushing the power button (not holding) will actually turn your computer back on - not wake it. Try that and see if it works.