Animals

Dolphins Sleep with Only Half Their Brain at a Time

Dolphins practice unihemispheric sleep, keeping one half of their brain alert while the other rests. This remarkable adaptation allows them to continue breathing consciously and remain vigilant for predators while still getting the sleep their bodies require. The wakeful side of the brain keeps the opposite eye open to monitor the environment.

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Unlike humans who sleep with both brain hemispheres at rest, dolphins can only allow one side of their brain to enter deep sleep at a time. The other half remains alert, controlling conscious breathing and maintaining awareness. As each hemisphere alternates sleeping, dolphins close the eye opposite the sleeping side of their brain. Studies show that dolphins can maintain this unihemispheric sleep pattern for extended periods, with one remarkable study documenting a female dolphin named Say performing complex tasks accurately for 15 consecutive days while engaging in continuous unihemispheric sleep. This adaptation evolved because dolphins, like all marine mammals, must surface to breathe and cannot afford to lose consciousness. The ability to sleep while remaining partly alert also provides protection against predators and helps dolphins maintain social cohesion within their pods.