Some of my fish go into such deep sleeps that I freak out, thinking they're dead. Yeah, if you really want to know, just leave all of the lights off at night for a couple of hours, then come back upstairs (or wherever) and flip on the lights. Odds are they'll be in their hides sleep, or just acting very sluggishly as they move around. Some people've noticed their fish almost seem to get blinded, like people do if they wake up at night and go turn on a light for whatever reason. I'll say with great belief that YES, fish sleep ^_^ And it's the cutest thing ever to see.