Why Mac Pro wakes up by itself?

I have a 2009 Mac Pro with 10.10.1 installed. The machine periodically wakes up by itself; no wake up time is set and I disconnect the keyboard to prevent accidental waking by the cats walking on it...they do that sometimes! ;-)

I see nothing in the console when I witness this waking..one person has suggested that periodic waking under OS X is normal because the computer is checking in with the router.

Anyone know about this waking by itself?

thank you!

ray