I have the same problem. Can you check (by opening "Console") whether you have an error message like this?
Code:
kernel[0]: The USB device Apple Internal Keyboard / Trackpad (Port 5 of Hub at 0x14000000) may have caused a wake by issuing a remote wakeup (2)
It seems that many users on this forum are experiencing this problem, on the Apple support forum there's a
thread about it as well.
It could be a hardware issue, for example a loose cable. I'm wondering whether all users have it (in which case a software issue seems more likely), or whether the group of users experiencing this problem is actually quite low. In that case I think it might be a hardware issue.