This happens whether or not I just close it up, leaving it in sleep mode, or shut the machine down entirely.

It's a Dell Inspiron, about a year and a half old. I have no idea how long this has been going on, never even noticed it until my girlfriend complained that it had been waking her up (it's kept on a desk fairly close to the bed) and now that she's pointed it out I can't help but notice it myself. Happens literally every single night between 3-4AM and when I open it in the morning nothing seems out of place I just get the start screen as I would had I left it in sleep mode.

At first I thought a software update, but I can't see one and I can't imagine it would continue nightly if that were the case. I'm at a loss as to what could be causing it (malware?) but am also a complete and total tech idiot. Any ideas?