How to figure out what process or event is waking up laptop from `xset dpms force off`

I have a laptop where I tried xscreensaver auto blanking screen after 1 minute of idle time and even though everything seems idle it doesn't blank the screen. So then I ran this command to blank the screen manually: xset dpms force off This works but after a minute or two the laptop wakes up and the screen lights up again. How do I debug this? I want to know what process or event is happening that is making the screen turn on again after I manually turn off the monitors. Perhaps it's the same thing that is preventing xscreensaver from blanking the screen also?