How do dolphins sleep?

Answer 1

half of a dolphins brain sleeps at a time while the other half tells them while to come back up to breathe

Answer 2

Dolphins have to be conscious to breath. This means that they cannot go into a full deep sleep, because then they would suffocate. Dolphins have "solved" that by letting one half of their brain sleep at a time. This has been determined by doing EEG studies on dolphins. Dolphins sleep about 8 hours day in this fashion.
A dolphin's behavior when sleeping/resting depends on the circumstances and possibly on individual preferences. They can either:
1. swim slowly and surface every now and then for a breath
2. rest at the surface with their blowhole exposed
3. rest on the bottom (in shallow water) and rise to the surface every now and then to breath.
Dolphins sleep very differently than most land mammals, in a way that dolphins are not totally unconscious when they sleep. Research has shown that only half of a dolphin's brain sleep at a time, leaving the other half fully aware of dangers that could possibly occur. Dolphins also need to be conscious to breath. This means that they can not be in deep sleep or else they will drown.
Dolphins sleep very differently than most land mammals, in a way that dolphins are not totally unconscious when they sleep. Research has shown that only half of a dolphin's brain sleep at a time, leaving the other half fully aware of dangers that could possibly occur. Dolphins also need to be conscious to breath. This means that they can not be in deep sleep or else they will drown.