Dolphins can't breathe underwater, so every time a dolphin needs to breathe, it has to make the decision to come to the water surface to breathe and supply its lungs with oxygen. Yet a dolphin might only be able to hold its breath for about 15 to 17 minutes. So how do they sleep?

Half of Their Brain At a Time

Dolphins sleep by resting half of their brain at a time. This is called unihemispheric sleep. The brain waves of captive dolphins that are sleeping show that one side of the dolphin's brain is "awake" while the other is in a deep sleep, called slow-wave sleep. Also, during this time, the eye opposite the sleeping half of the brain is open while the other eye is closed.

Unihemispheric sleep was thought to have evolved due to the dolphin's need to breathe at the surface, but may also be necessary for protection against predators, the need for toothed whales to stay within their tightly-knit pods, and for regulation of their internal body temperature.

Dolphin Mothers and Calves Get Little Sleep

Unihemispheric sleep is advantageous to mother dolphins and their calves. Dolphin calves are especially vulnerable to predators such as sharks and also need to be near their mothers to nurse, so it would be dangerous for dolphin mothers and calves to fall into a full deep sleep like humans do.

A 2005 study on captive bottlenose dolphin and orca mothers and calves showed that, at least when at the surface, both mom and calf appeared awake 24 hours a day during the first month of the calf's life. Also during this lengthy time period, both eyes of the mom and calf were open, indicating that they weren't even sleeping 'dolphin-style'. Gradually, as the calf grew, sleep would increase in both the mom and calf. This study was questioned later, as it involved pairs that were only observed at the surface.

A 2007 study, though, showed a "complete disappearance of rest at the surface" for a minimum of 2 months after the calf was born, although occasionally the mother or calf were observed with an eye closed. This may mean that dolphin mothers and calves engage in deep sleep in the early months after birth, but it is for only brief periods. So it appears that early in the dolphin's life, neither mothers nor calves get much sleep. Parents: sound familiar?

Dolphins Can Stay Alert for at Least 15 Days

As mentioned above, unihemispheric sleep also allows dolphins to monitor their environment constantly. A study published in 2012 by Brian Branstetter and colleagues showed that ​dolphins can remain alert for up to 15 days. This study initially involved two dolphins, a female named "Say" and a male named "Nay," who were taught to echolocate to find targets in a pen. When they identified the target correctly, they were rewarded. Once trained, the dolphins were asked to identify targets over longer periods of time. During one study, they performed the tasks for 5 days straight with extraordinary accuracy. The female dolphin was more accurate than the male—the researchers commented in their paper that, subjectively, they thought this was "personality-related," as Say seemed more eager to participate in the study.

Say was subsequently used for a longer study, which was planned for 30 days but was cut off due to an impending storm. Before the study was concluded, however, Say accurately identified the targets for 15 days, demonstrating that she could perform this activity for a long period of time without interruption. This was thought to be due to her ability to get rest through unihemispheric sleep while still remaining focused on the task she needed to perform. The researchers suggested that a similar experiment should be done while also recording the dolphins' brain activity while the tasks are being performed to see if they engage in sleep.

Unihemispheric Sleep in Other Animals

Unihemispheric sleep has also been observed in other cetaceans (e.g., baleen whales), plus manatees, some pinnipeds, and birds. This type of sleep may offer hope for humans who have sleep difficulties.

This sleep behavior seems amazing to us, who are used to — and usually need to — fall into an unconscious state for several hours each day to recover our brains and bodies. But, as it was stated in the study by Branstetter and colleagues:

"If dolphins sleep like terrestrial animals, they might drown. If dolphins fail to maintain vigilance, they become susceptible to predation. As a result, the apparent 'extreme' capabilities these animals possess are likely to be quite normal, unspectacular and necessary for survival from the dolphin's perspective."

Have a good night's sleep!