Unihemispheric Sleep: How Do Marine Animals Doze Off Underwater?

If terrestrial mammals want to sleep, they would close their eyes and drift away for the night as long as they keep themselves from danger. However, sleeping is not this easy for their marine counterparts since they need to surface for air immediately. They also cannot float at the water's surface and slumber, or they will be exposed to predators and heat loss.

So, how do these creatures put themselves to sleep? One way is through a mechanism called unihemispheric sleep.

Half-Brain Shut Down

Unihemispheric sleep is a sleeping style in which one half of the brain is in resting mode while the other half remains alert. This is in contrast to normal sleep, in which both eyes are shut and both hemispheres of the brain are unconscious. It allows marine mammals, like dolphins, to rest in open water.

According to biologist Patrick Miller from the University of St. Andrew, this form of sleep is valuable for ocean-dwelling mammals because it enables them to maintain a low activity level while still sleeping half of their brain at a time.

Dolphins are the well-studied marine mammals capable of this sleeping style. Brain scans conducted on captive dolphins reveal that one hemisphere of the brain is in slow-wave, deep sleep while the other hemisphere is alert. This allows them to sleep with one eye open literally.

This sleeping style is common but not unique to cetaceans, mammals that include whales, dolphins, and porpoises. Many species of birds are also known to use unihemispheric sleep, which enables them to doze in the midst of flight.

Miller noted that birds and dolphins use the unihemispheric sleeping technique for different purposes. For example, many birds on the outside of a flock keep the eye facing away from the group open to watch for predators. Dolphins do the opposite, as the open eye faces the rest of the pod to avoid being separated from the group.

Other Sleeping Technique

Experts noted that not all cetaceans are capable of a half-brain sleeping technique. Some use bihemispheric sleep, where both brain hemispheres fall asleep, just like humans and other mammals do.

Some examples of bihemispheric sleepers are the northern elephant seals. A 2023 study reveals that the seals dive to a depth of about 1,000 feet (300 meters), where their brains slow down and they enter rapid eye movement (REM) sleep. Once their REM cycle starts, the seals flip upside down and spin in a slow circle while continuing to doze off.

Meanwhile, sperm whales sleep by making shallow dives beneath the water's surface, slowing their swim speed to a stop, and pitching lazily upward. While they are sleeping, whole pods of sperm whales can be seen nose-up, just below the surface of the ocean. At this point, the whales are completely unresponsive, which indicates that they are in a deep sleep.

However, the sperm whale rests for about 20 minutes underwater before returning to the air. Once it has caught its breath, the sperm whale slinks below the surface for more rest. This behavior may continue for up to three and a half hours.