My male is a very sound sleeper, and usually will stay asleep for quite a while after I turn the lights on. He is very still, usually at the very bottom of his tank. Sometimes he is on his side, completely flat, other times he is partially propped up. He almost looks dead when he is like that, but I can see his gills moving. After a while, he wakes up and pops up to the top looking for food.
My female's belly becomes a very pale silver color when she sleeps, and she is also very still, she sleeps anywhere. She does not sleep as soundly as my male, but when she is startled awake, she is very disoriented and tends to bump into things.