Dolphins Can Sleep With Half Their Brain Awake and There’s a Very Good Reason Why

For humans, falling asleep means checking out for a while.

For a dolphin, that would be a serious problem.

Dolphins live their entire lives in the water, but like us, they’re mammals with lungs. They need oxygen from the air, which means regularly returning to the surface to breathe.

So what happens when it’s time to sleep?

Dolphins came up with one of the strangest solutions in the animal kingdom.

They essentially sleep with half of their brain at a time.

Scientists call it unihemispheric slow-wave sleep. While one hemisphere of a dolphin’s brain enters a sleep-like state, the other remains more alert. Later, the two sides can switch roles.

That allows the dolphin to get the rest its brain needs without completely disconnecting from the world around it.

And when your bed is the ocean, that’s pretty important.

Unlike humans, dolphins don’t breathe automatically in quite the same way we do. Their breathing requires enough awareness to reach the surface and open their blowhole at the right time.

Completely losing consciousness for eight hours obviously isn’t an option.

By keeping part of the brain active, dolphins can continue swimming slowly, periodically surface to breathe and maintain some awareness of what’s happening around them.

They can even literally sleep with one eye open.

Research involving bottlenose dolphins and beluga whales has found a relationship between which brain hemisphere is sleeping and which eye is closed. The eye connected primarily to the sleeping hemisphere tends to close, while the eye associated with the more active hemisphere is more likely to remain open.

It sounds like something out of science fiction.

For a dolphin, it’s bedtime.

Sleeping Without Completely Switching Off

Breathing isn’t the only reason staying partially alert can be useful.

The ocean doesn’t suddenly become safe because a dolphin is tired.

Predators may still be nearby. Other members of the group can move. Currents and environmental conditions can change.

Unihemispheric sleep allows dolphins to maintain some ability to monitor their surroundings while still getting necessary rest. Researchers studying the phenomenon have suggested that it may serve several functions, including maintaining breathing, movement, body temperature and vigilance.

Some dolphins even continue moving while they rest.

Spinner dolphins around Hawaiʻi provide a particularly good example. After spending much of the night feeding offshore, groups return to protected nearshore waters during the day to rest.

To someone watching from a distance, the dolphins may still appear to be swimming.

They are.

They’re just doing it slowly while portions of their brains rest.

That’s one reason NOAA asks people to give resting spinner dolphins plenty of space. An animal doesn’t necessarily have to be motionless with its eyes closed to be sleeping, and repeatedly disturbing resting dolphins can interfere with an important part of their daily lives.

Scientists have confirmed dolphin sleep patterns using electroencephalograms, or EEGs, which measure electrical activity in the brain. Those recordings have shown the distinctive pattern of slow-wave sleep appearing primarily in one hemisphere while the other remains in a more wake-like state.

Researchers have also found that the hemispheres can alternate, allowing both sides of the brain opportunities to rest.

Dolphins aren’t completely alone in this ability.

Other marine mammals, including some seals and manatees, can show forms of unihemispheric sleep. Scientists have also documented it in birds, where keeping part of the brain alert may help an animal remain aware of predators while resting.

But dolphins provide one of the clearest examples of just how dramatically sleep can evolve when an animal’s environment demands it.

Humans can climb into bed, close both eyes and disappear from the world for hours.

A dolphin doesn’t have that luxury.

It still has to swim.

It still has to breathe.

It still has to know, at least to some degree, what’s happening around it.

So evolution arrived at an extraordinary compromise.

Half the brain gets some sleep.

The other half takes the night shift.

Sources: NOAA Fisheries