How do dolphin sleep since they they need oxygen but live in water?
Porpoises (the mammal dolphin, not the fish dolphin) have the unique ability to sleep with one half of their brain inactive while the other half remains active. They periodically rise to the surface to breathe and obtain fresh oxygen during their sleep cycle. I do not think that it has been determined yet that when they rise to the water surface both halves of their brain become temporarily active, or one side remains inactive.
Dr John Cowsar
San Antonio, Texas USA
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Update: November 2011