Why do dolphins sleep with one eye open?

Dolphins cannot sleep the way humans do because they must consciously surface to breathe. Instead, they use unihemispheric slow-wave sleep, in which one half of the brain sleeps while the other remains awake enough to control breathing and monitor the environment. The eye opposite the awake hemisphere is often open, which is why it can look as if the dolphin is sleeping with one eye open. This adaptation helps them avoid drowning and stay responsive to predators or social signals. It is also seen in some other marine mammals and birds.