My observation on Mandarins during sleep is that their color go drabby almost pale. They curl up in semi circular fashion and are typically found nestled against a piece of rock or the corner of the tank.
If you shine a light on them, you actually stun them and they do not swim away. This is the best way to catch a fish by the way. Before the lights go on, have a net ready and once the lights go on the fish are stunned/blinded and because they are sleepy are too groggy to react and swim away.
I have caught many difficult fish this way and it eliminates a lot of stress then chasing them around the tank when they are fully awake.
You also might find a film of silk around them; this is secreted from their body to hide their scent from predators just like the six line wrasse does.
This is purely my observation only and the opinions are my own. I am not replying to this thread to promote my business.
