WHen do Turtles sleep?

Frank, the turtle tends to look like he is dead sometimes. He just floats in the water, not moving. If you tap the cage even, nothing. Well I did some research and in fact turtles do sleep. Painted turtles particularly like to sleep under water. They sleep buried in the sand or mud at the bottom of their habitat. They can breathe air and also absorb oxygen in water. We don't have any sand or mud in our little aquarium but we did put in a ghetto Cave for him and he does like to sleep under there. I guess it gives him the impression that he is buried. He probably feels safe. He is an interesting little creature that Frank. Anne has taken to putting him in her prayers at night, so I am sure now that a 3and a half year old asks for God's blessing on him, he will live a long fruitful life. And I can stop worrying that he is dead.