Powder Blue Tang Sleeping Habit

I bought a small (1" inch) Powder Blue Tang yesterday for my tank. I set everything up and introduced it to my tank and all was well.

I woke up early this morning to see how he was going and I found Him lying on his side on the bottom of the tank in a little pocket of live rock. I looked closely at the fish and it was breathing.

I put my hand in the tank to help the fish and he sprung up and started swimming around as normal. I feel really bad that I woke the fish up.

My question is do the Powder Blue Tangs sleep this way, cause if they do, a crab is sure to get him when he least expects it and I will loose him.

Cheers
Goong