shut down instead of sleep ?!?

for some reason, when my computer goes to sleep, it shut downs completely?!?!? and i checked the "restart after power falure box" and it restarts everytime it shuts down due to it not being able to sleep???
I am runnin an Imac g5 with OSX 10.4 Latest updates.

This happened to me one time with my PowerBook when I closed the display, but after I restarted, it didn't happen again. I have no idea what caused that one time... I just figured it was a freak thing.

Anyone else here that might know a little something about this phenomenon?