Dolphins are conscious breathers: they actively decide when to come to the surface to breath. This make sleeping very difficult, as what happens if the dolphins go into a deep sleep? Probably they would suffocate.
Happily, this does not happen. They have their own method to get their zzz’s. Dolphins do not go into a deep sleep: they let half of the brain to sleep at a time. The other half of the brain reminds dolphins to go to surface to breathe.
Dolphins sleep in this fashion around 8 hours a day. Sometimes they can get even more sleep if they spend their resting time floating at the surface with their blowhole exposed.
See more about how dolphins sleep on this Youtube video: