How do fish sleep?

I have never really been one to keep fish as pets. My parents always had fish, but I never paid much attention to them. A little bit of background: As some of you might already know, last year we installed a pond in our yard and we have koi. Someone gifted my husband a fantail goldfish to add to the pond this year, saying that he would be just fine. He seemed to enjoy the pond itself, but the koi kept attacking him and trying to eat him. So needless to say, this fish now lives in a tank in our bedroom.

It took him a few weeks to heal up, but he seems very happy now. The thing is, every now and then I will catch him sort of aimlessly floating around in the tank. Not upside down, per se, but on his side or with his head pointing down. I always panic and think he is sick or dying.. but usually within moments he will “snap out of it” and start swimming around, business as usual.

Could he be sleeping??