Hibernation Issue

Recently I installed windows7 64bit home edition on my desktop pc, at night before I go to bed I like to place the PC into hibernation mode, however ever since I installed the new windows the PC will hibernate but about 15-20 mins later the computer just restarts itself. Is this a known bug? Or any remedies? Thanks!

I have updated my graphics card driver which I thought was the fix, because I waited over an hour and the PC was still in hibernation, however after I went to bed and I got up in the morning the PC was awake again...