Aquatic mammals, birds and reptiles can sleep with one eye and one eye. The brain that controls the open eye does not actually fall asleep and is still awakened. The latest research found that humans also have a similar half-brain sleep pattern.
Decreased vigilance during sleep can cost animals a heavy price. Therefore, in order to reduce the risk caused by sleep, some of the animals have the ability to overcome the risk of sleep during the evolution process. One of them is to reduce the depth of sleep. Some animals have evolved the ability to sleep half asleep, that is, only one side of the hemisphere is in sleep. The other side remains awake.
EEG studies have shown that in the sleep state, the cerebral hemisphere meets the typical sleep EEG performance, while the other side of the cerebral hemisphere in the awakened state exhibits typical arousal EEG features. The study found that this unilateral brain sleep unilateral arousal sleep pattern is very common in the biological world, from birds, reptiles, to mammals, especially marine mammals such as whales.