How do mammals sleep underwater?

Some marine mammals like dolphins choose to sleep with half their brains off while sperm whales sleep upright in the ocean.

Sperm whales sleep upright for short periods. Photo: Franco Banfi — Sperm whales sleep upright for short periods. Photo: *Franco Banfi*

If cetaceans want to sleep, they can't just close their eyes and drift off at night, because they need to come to the surface mid-flight to get air. They also can't float and fall asleep because of the risk of predators and hypothermia, according to Live Science .

One solution is to shut down one half of the brain at a time. Called unihemispheric sleep, it’s how cetaceans like dolphins rest in the open sea. “Unihemispheric sleep is really useful for these animals because it allows them to maintain low activity levels while still sleeping one half of their brain at a time,” says Patrick Miller, a biologist at the University of St Andrews in the UK.

Dolphins are the most studied cetaceans known to sleep this way. Brain scans of captive dolphins show that while one hemisphere is in deep slow-wave sleep, the other hemisphere remains awake, allowing the animal to sleep with one eye open. This sleep pattern is common among cetaceans, the group of mammals that includes dolphins, whales, and porpoises. Many birds also use unihemispheric sleep to rest while flying.

But according to Miller, birds and dolphins use hemi-brain sleep for different reasons. For example, in a flock of birds, many of the birds on the outside of the flock keep their eyes open on the side away from the group to keep an eye out for predators. Dolphins do the opposite. When they sleep, they often keep their eyes open on the side facing the group, likely to avoid being separated.

Not all cetaceans are monohemispheric. Some use dihemispheric sleep, where both hemispheres of the brain sleep, like humans and most other mammals. “It’s really hard to measure brain activity in marine animals that you can’t capture, like sperm whales, blue whales, or humpback whales. In those cases, behavioral data are the best clues to their sleep behavior,” Miller says.

Researchers can then tag animals to track their behavior. A 2008 study by Miller used tags on sperm whales ( Physeter macrocephalus ) to show that they sleep in mid-ocean in short bursts. Sperm whales dive near the surface, slow down, then stop and sleep upright. Their upright sleeping position is likely due to a buoyant oil called spermaceti in their heads.

During sleep, entire pods of sperm whales keep their heads up, near the surface. During this time, the animals remain completely unresponsive, indicating that they are in a form of deep sleep. However, sperm whales can only sleep for 20 minutes underwater before having to surface for air. After the whale has finished breathing, it sinks back down to rest for more time, and can continue this behavior for up to 3.5 hours.

Northern elephant seals ( Mirounga angustirostris ) also sleep with both hemispheres in similar short shifts. A 2023 study by Jessica Kendall-Bar, a postdoctoral fellow at the Scripps Institution of Oceanography at the University of California, San Diego, was the first to measure brain activity in sleeping marine mammals. Kendall-Bar and colleagues found that seals dived to depths of about 300 meters. There, their brain activity slowed down and they entered rapid eye movement sleep. They flipped upside down and rotated in slow circles as they continued to sleep.

Due to the risk from predators, elephant seals limit their total sleep time at sea to about two hours per day, making them among the shortest sleepers of all mammals.

An Khang (According to Live Science )

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