They sleep just like a shark. If a shark stops moving through the water, they die.
Not all animals have human like sleep/wake schedules.
Is that true about sharks, that they are in constant motion, and must remain that way? I didn’t know.
http://www.elasmo-research.org/education/topics/b_40_winks.htm It had been known for centuries that some sharks — such as nurse, wobbegong, angelsharks, and batoids such as skates and stingrays — spend most of their time lying motionless on the bottom actively pumping water over their gills. ... Clark speculates that the enhanced oxygen content may make breathing easier for the motionless sharks and may produce a narcotic effect the sharks enjoy.
also see: The Sleeping Sharks of Yucatan