Norm, has dug a large hole in the corner of his tank and sometimes he goes into it and kinda' lays there. He's been doin' it for a long time, almost like he's depressed. Here he is...
My big one dug holes in the back of the tank and so did my other larger one I used to have but I never saw them sleep there. Have you checked you nitrates?
Norm looks pretty "normal" to me in those pics. He's not actually resting on the bottom (which is abnormal behaviour). He seems to have just picked a spot to his liking.
If he is actually laying on the bottom, I'd be suspicious of gut parasites, which is what I've seen cause that sort of behaviour before.
Fish also one of alive animal. So, they need to take a rest for energy as like as other animal. As you known that there are not eyelid in fish, so we can't see that they are sleeping but they used to take a rest at the bottom of the tank without any movement.
The another one may be due to over feeding. If you feed many of food to him, his stomach fill with food and tide. At that time, they used to lie at the bottom of tank or put their head at the corner of tank.
So.........