Dolphins Sleep with One Eye Open

A dolphin's aquatic life presents a unique challenge: the need to breathe consciously. Unlike humans who breathe automatically, a dolphin must decide when to surface for air. Going into a deep, unconscious sleep like we do would lead to drowning. To solve this, dolphins have developed a remarkable sleep method called "unihemispheric slow-wave sleep," where one half of their brain rests while the other remains awake and alert. This allows them to continue swimming, watch for predators like sharks, and crucially, to keep breathing. While one hemisphere is in deep sleep, the corresponding eye is closed; for instance, the left eye will be closed when the right side of the brain is sleeping.

This incredible adaptation means dolphins are never fully unconscious. They can rest while still maintaining essential bodily functions and awareness of their environment. While sleeping, dolphins might swim slowly and steadily near the surface or rest motionless on the water (Review). In shallower areas, they have even been observed sleeping on the seabed and rising to the surface for air. Scientists who have studied this phenomenon using electroencephalography (EEG) have confirmed that one side of the brain shows the characteristic tracings of deep sleep while the other shows wakefulness. This unique sleep pattern is not limited to dolphins; other marine mammals like seals and whales also exhibit it.

Interestingly, this state of partial sleep seems to be so effective that dolphins may not experience REM sleep, the stage where humans typically dream. One theory is that since their brains are never entirely at rest, they don't need the "warming up" function that REM sleep provides for fully sleeping brains. Furthermore, newborn dolphin calves and their mothers show an even more extreme behavior, forgoing sleep almost entirely for the first few weeks of the calf's life. This constant motion helps the calf, who has not yet developed enough blubber, to stay warm and avoid predators.