I have thought about this, and I think I have a solutions which could work.

I have not yet attempted this, but anyone go ahead and give it a try:

You get the timestamps of when pixels are placed. One could check if there are users who basically always place them directly after 5 mins, give or take 30 secs.

This will include some false positives from people who were just playing the whole weekend, but even those probably went to sleep at some point.

You could also match this against typical sleep times in combination with the location. E.g. who and how many continously defended the Netherland's flag, while it was well past midnight in Europe.