Here I am sitting on my bed with my laptop (Mac Pro turning on randomly)

[Ravich]

And I can hear my Mac Pro waking itself up from sleep every so often. What does this mean?

[SpookyLars]

Try to uncheck option "Allow Bluetooth devices to wake this computer" in System Preferences/Bluetooth/Advanced.

[kez]

time machine?

[DualShock]

If you also have an AirPort Extreme / Express, and turned on Wake On Demand:

http://support.apple.com/kb/HT3774

In that article:

Macs that have Wake on Demand enabled will occasionally wake for a brief time, without lighting the screen, in order to maintain registrations with the Bonjour Sleep Proxy.

[Weepul]

This happened to me when I had my USB printer sitting atop a small refrigerator. My hypothesis is that the interference it produced when it started cooling was received by the printer and the computer interpreted this as activity on the USB, waking up the computer.

In other words, check if any attached devices may be malfunctioning.

[Ravich]

I'm not positive but I think it might have been because Logic was running.