Are you running Snow Leopard?
I have a Mac Pro and since upgrading
to SL my Mac goes to sleep very early
and takes quite a bit of key pressing and
mouse movement to wake it back up.
Have read similar complaints from SL
users about this.
For now, I just go into PREFERENCES
and extend the time I want for the
computer to go to sleep.
I am sure if this is a SL issue that it
will be resolved in future updated.