Recently I wanted to leave the PC asleep and let it wake up to do a defrag scheduled for 2 am. I switched it to sleep mode, but it instantly woke up again. I've sinced tried from time to time to get it to sleep but it won't stay put. It's not as though there's a scheduled task about to start just at that moment. Anyway, I've checked that one. Anyone got an answer? I use Vista Home Premium, and an nVidia GeForce 8800 GTS card which at one point I heard might be the problem, though I'm doubtful.
I use Vista, which has discarded Standby and Hibernate. They are replaced by Sleep and Hybrid Sleep (Hybrid Sleep saves the status on the hard disk as well as to memory).
Nothing has been set to wake it up. I don't care if it's the KB, Mouse or Power Button, but I have been careful not to let, say, the mouse wake it up instantly, i.e. I don't touch anything after teeling it to sleep.
Unfortunately, as with many issues, Vista Help & Support uses loads of words, but somehow fails to leave one with a firm answer.