They have no eyelids, and need a dark tank in order to sleep, wich is why you should have the lights on for 12-14H a day and turn the lights out at schedualed times and not turn them back on after that.
All my fish sleeps and maintains reflex control of their 'balance', but I had seen one goldfish completely out and just floating around. I thought it was dead; turned on the light and it woke up and swam away.
Dolphins sleep by having half their brain sleep as they swim. They will switch what half is sleeping throughout the day. This way they are not completely unconscious like us, humans. kind of neat.
my red devil is like a statue at about 6am in the morning, it normally flares at me whenever it sees me but at this time... even if i put my face in the tank.. its just there motionless... when i make a sound it swims away fast as if startled then once it realizes that its me it starts flaring again..
with regards to the sharks.. they do sleep i think.. i saw some documentaries in nat'l geographic where divers visit some shark dens... underwater caves... and the sharks there are motionless and out of it.... just lying around.. like resting or sumthing...
maybe they dont have to move to breathe sufficient oxygen since animals usually have smaller O2 requirements while sleeping bec. some body functions are shut down.. but im no zoologist so its just a maybe