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A few weeks ago, I was at the zoo with my friend and somehow the topic of how dolphins sleep came up. Because dolphins and whales are mammals, they are required to breath oxygen and therefore surface the water often in order to breath. So, how can a dolphin or whale possibly be able to sleep if they constantly have to surface? This is really fascinating! Apparently, dolphins only sleep with half of their at a time. They will shut down half of their brain along with the opposite eye. The other half of the brain is awake, but at a reduced level of alertness. This allows the dolphin and whale to be able to watch for predators as well as rise to the surface for air. It has been estimated that after two hours, the dolphin or whale will swap sides. Little is known about whether these sea mammals have REM sleep, but in the pilot whale, it was estimated that it had about six minutes of REM sleep in a night.