My bettas enjoy sleeping inside some of their ornaments, most likely because they feel safe there.
When angelfish sleep, they often begin to tilt sideways. I've tried to take some pictures but when I approach the tank, they often wake up.
In the mornings when the automatic timing turns on the lights, I find most of the tetras are asleep along the bottom of the tank. They slowly begin to wake up and move throughout the tank in a school until they know their world is safe.
How do fish know when there is danger when they sleep? Along the sides of their bodies, they have receptors that detect movement in the water. Those receptors wake up the fish if they are sleeping.