Go to step #4 in this tutorial:
How to Troubleshoot a Vista or Windows 7 Sleep Mode Problem
Then open a command prompt (cmd) and type/copy/paste
powercfg lastwake at the blinking cursor, press enter. The line that you're interested in is Description, that will tell you what was the last device to wake your machine. It may be possible to invoke this cmd every time you see your machine awake in order to create a list of what is waking it.
Most everyone sees the mouse, keyboard, and an audio or network card, maybe you have a balky mouse or a stuck key on your board.
It may be easier though to use step #5 to see all the devices that could wake your machine and then go over each devices settings, be aware that you will need an elevated cmd prompt for this step.
There is also a possibility that a task such as, an overnight malware scan could be awakening your machine.
Do you use a screensaver? try turning it off.
The tutorial is pretty comprehensive and gives links and tips to help like the screensaver link in step #7, and how to open cmd prompts.
Just saw TZ's posting, yep, there can be behind the scenes activity, and I follow his advice; "If I'm not usin' it I shut it down".