Have a couple pairs of Picassos. With one pair, when all the lights are off, one "sleeps" on the sand bed and the other near the top of the waterline near the overflow grate. Both sleep on their sides and semi-float, flapping every so often.
As others have said, it freaked me out the first time I saw this when the lights came on and I immediately panicked and thought I had two sick fish. Come to find out that this is their "normal sleeping" behavior.
Normal behavior definitely varies for every clownfish, but they are always fun to watch. One pair probably thinks they are part dog because every time they see someone beside the tank, they'll hover near the waterline because they assume they are getting fed... again.