Vista - Hibernate mode turns to sleep mode after a few hours

I just purchased an hp dv2500 laptop and have tried to put it in hibernation,
but it only lasts for a few hours. When I put my computer in hibernate the pc
completely turns off, (no lights flashing from the power button or anything)
but when I come back a few hours later, I notice that the power button is
flashing and my computer is in sleep mode. When I open the lid, my computer
immediately turns on, I don't need to hit the power button or anything. It
doesn't wait a few moments to return to the desktop like it should after
hibernation. It continually runs my battery down, which is what i don't want
when it's supposed to be in hibernation.

Please help. I'm confused.