When I put my PC (running Windows XP) into hibernate mode at night, I find that it spontaneously awakens by the morning.

That happens to me sometimes too. It puzzled the hell out of me until I noticed the cat hairs on the keyboard.

It can be for various reasons
1. It could be your mouse’s settings, where it can wake your system with just a small movement and as noted by darkscribe your keyboard could be a reason.
2. Some programs may have the ability to wake your pc without you knowing
3. Automatic Update
4. Your network card(check it’s power management)
5. Check your BIOS Power Management.

Hibernate and Standby modes should not be easily awoken by keyboard strikes or mouse movement; only the power button.

Like @MrGeneVan, I would guess at #2 and #3 on his list. Unfortunately, unless you’re super adept at scanning the Windows Registry, or if you just happen to remember what app might be responsible, I’m not sure it’s going to be easy to track this down. Do you have any idea at what time the machine wakes up?

That, or you’re compromised by some malware and your pc is actually part of a sinister bot network. (Wish I was joking there. :-\)

I jinxed myself. It is winter here and cold (for us 24˚C/75˚F). I just walked in and found the cat curled up on the nice warm keyboard of the laptop again. I forgot to close the lid.

some bios’ allow a computer to turn back on automatically after a power outage without any user interaction. so, it might be a power issue…you might be having brownouts or something like that. try plugging it directly into the wall or a different surge protector.

i’ve never tested this with hibernation, so i don’t know if the bios setting would affect the system the same way.

I don’t know what sort of systems you use there, but here we use a modulated pulse on the AC power lines to switch “off-peak” hot water systems, swimming pool pumps etc., on and off. Ours comes on at around 11:00 pm, and off around 4:00 am. That has been known to wake computers, but not any that I have owned.