Most sharks need to constantly swim in order to breathe and cannot sleep very long, if at all, or they will sink.
Be a shark. The day you stop swimming is the day you start slipping and let others catch up to you. Don't get eaten up by a school of Rookie-ass guppies just because you wanted to relax. And in order to be at the top of the food chain, you must prove yourself to be a superior predator. Must be Frightening yet fascinating. Be a shark. Be a threat.
Some sharks sleep with only one brain hemisphere at a time, thus maintaining enough consciousness to breathe and to watch for possible predators and other threats.