If marine mammals want to sleep, they can't just close their eyes and drift away for the night, as they need to intermediately surface for air. Nor can they float at the water's surface and slumber, because that exposes them to predators and heat loss. So how do marine mammals get any shut-eye without putting themselves at risk?
One solution is to shut down one half of their brain at a time. Called unihemispheric sleep, it's one way marine mammals, such as dolphins, can get some rest while in open water.
"Unihemispheric sleep is really valuable to these animals because it allows them to maintain a low level of activity while still sleeping half of their brain at a time," Patrick Miller, a biologist at the University of St Andrews in the U.K., told Live Science.
Dolphins are the best-studied marine mammals capable of this style of sleeping. Brain scans on captive dolphins show that while one hemisphere is in slow-wave, deep sleep, the other hemisphere is alert, allowing the animals to literally sleep with one eye open. This style of sleeping is common in cetaceans — the group of mammals that includes dolphins, whales and porpoises — but it's not unique to them. Many bird species are known to use unihemispheric sleep, often allowing them to doze while flying.