Vista shutting down during sleep

Hi all,
I have Vista Ultimate 32bit.
I've been using sleep, and it's been working great, for several months.
All of the sudden, during long sleep periods, I cannot wake the machine and it turns out it had shut down at some point during the sleep.

If I put it to sleep and try to wake it after a few minutes, it wakes fine.
If I let it sleep all day, while at work, or all night I have to actually power it back on as it is no longer in sleep mode.

I've checked the logs and see nothing unsual in there, I've verified that the settings for sleep have not changed, hibernate is not on, the disk goes to sleep before the PC, etc...

I don't remember adding any software before this started happening and I definitely haven't changed my hardware or BIOS settings.

Hopefully someone has some suggestions on what I might try.
Thanks!

Well, I figured this out.
In my case something on the network was waking this machine up via Wake On LAN.
I disabled that and now it sleeps and wakes normally.