sleep

So I woke up and found one of my Angels between a rock and my tank glass. Is it normal for them to to that during the night to sleep or am I right to have moved him? It is 4am so it would be time for him to sleep, if they sleep that is. I'm going to move the rock farther from the glass. But if I...

I am a novice and just wondering do fish sleep and if so how ? Thanks for you replies