As mammals that need to periodically surface for air every few minutes, dolphins can't sleep without risking drowning. Dolphins and other cetaceans, like whales, have instead evolved a remarkable adaptation. Each creature can turn off half its brain, remaining conscious while essentially being half asleep, according to LiveScience.
Technically called "unihemispheric slow-wave sleep," when dolphin brains power down to 50 percent, they close the opposite eye (leaving one open to scan for danger) and enter what amounts to dolphin autopilot to rest while still remaining conscious of their surroundings. Unlike virtually every animal in the world, sleeping dolphins can still control their movements, swimming slowly, surfacing for air, and scanning the environment for threats.
It's basically impossible to ever deprive a dolphin of sleep, and studies have shown that no amount of sleep deprivation as we understand it impacts a dolphin's ability to respond to stimulus. In one experiment done by the U.S. Navy, two dolphins were trained to respond to a series of beeps that were then played over the course of the next five days at random times. At no point during the experiment did the dolphin's reaction time ever slow or diminish, even when it was "asleep." Dolphins are always ready and always watching. Always.
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