Dolphins in the wild sleep with one eye open and almost never go into full unconsciousness. By allowing only one brain hemisphere to go into slow-wave sleep at a time, a dolphin can stay alert enough to watch out for predators and control their breathing. Unlike humans, dolphins have to actively decide to breathe, which is another reason why they need to stay partly awake.
More facts about dolphins:
- Dolphins remain in their pseudo-sleep state for about eight hours out of the day.
- In captivity, dolphins actually do go into full unconsciousness, though they still have a tail reflex to keep their blowholes above water. When they enter full unconsciousness, their breathing becomes automatic.
- Dolphins aren't the only ones to sleep this way — whales also sleep with only one hemisphere of the brain at a time.