MacBook Pro spontaneously wakes from sleep after upgrading to Leopard?

For some reason, after I upgraded to Leopard, then whenever I put my
MacBook Pro to sleep, it spontaneously wakes up after some time (can
be from a few minutes to an hour or two.) I don't know why this is
happening, but I know for sure that it's related to Leopard since I
put my laptop to sleep every night (it's in my bedroom) and after
upgrading to Leopard, it started to just wake up a short while after
putting it to sleep.

At first I thought it could be Time Machine, so I disconnected all my
external drives, but that didn't seem to work.

Does anyone know why this is happening?
Thanks in advance!