Hot Heads: Why Mammals Need R.E.M. Sleep

Fur seals can switch off and on R.E.M. sleep when they're in the sea and on land, a pattern never seen before. Scientists believe it might mean R.E.M. sleep helps generate heat in our skulls, like shivering for the brain.
Credit...Mickrick, via Getty Images

On a December evening in 1951, Eugene Aserinsky, a physiologist at the University of Chicago, placed electrodes on the scalp of his 8-year-old son, Armond, before putting him to bed. Then the scientist retired to another room to watch a row of pens quiver across a rolling sheet of paper, recording the electrical activity in the boy’s facial muscles.

Hours later, the pens started to swing wildly. To judge from the chart, it seemed as if Armond were awake, his eyes darting about the room. But when Aserinsky looked in on him, his son was fast asleep.

Aserinsky had discovered R.E.M. sleep.

Eventually he and other researchers learned that during this state, the brain shifts from low-frequency to high-frequency electrical waves, like those produced in waking hours. When Aserinsky woke his subjects from R.E.M. sleep, they often reported vivid dreams.

Almost all mammals experience R.E.M. sleep, but even today researchers debate why it exists. On Thursday, a team of American and Russian researchers reported that fur seals may provide an important clue.

While they swim, fur seals switch off R.E.M. sleep entirely. It returns when they come back to land — a pattern never seen before.

Jerome M. Siegel, a sleep expert at the University of California, Los Angeles, and a co-author of the new study published Thursday in Current Biology, said that the seals provide evidence that our brains switch to R.E.M. sleep from time to time to generate heat in our skulls.

“R.E.M. sleep is like shivering for the brain,” he said.

Many scientists have argued that our brains require R.E.M. sleep each night to function properly. One clue comes from experiments in which researchers deprive rats of R.E.M. sleep for a few days.

As soon as the rats can sleep normally again, they experience a “rebound,” spending more time each night in R.E.M. — as if they need to catch up.

Some studies have suggested that the brain needs R.E.M. sleep to keep its metabolism in balance. Rats deprived of R.E.M. will eat more, and yet they also will lose weight.

This disruption can be lethal. “If you deprive rats of R.E.M. sleep, they’ll die in two weeks,” said Dr. Siegel.

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But other findings have raised doubts about the importance of R.E.M. Certain types of antidepressant drugs reduce R.E.M. sleep in users, for example, without evidence of harm.

R.E.M. isn’t even essential for dreaming. Researchers have found that people also dream during periods of so-called slow-wave sleep.

Some of the most puzzling evidence about R.E.M. sleep has come from the sea.

In the 1970s, a Russian biologist named Lev M. Mukhametov placed electrodes on the heads of dolphins. He discovered that they can put one side of the brain to sleep as they swim while the other side remains alert. Then they can switch, putting the other hemisphere to sleep.

But as hard as Dr. Mukhametov and his colleagues looked, they never found a dolphin in R.E.M. sleep.

In the 1990s, Dr. Siegel and Dr. Mukhametov started collaborating on studies of relatives of dolphins and found the hemisphere-switching sleep pattern in other species, such as gray whales.

More recently, the scientists wondered what they might find if they looked at a species between the two ends of the spectrum: a mammal that regularly slept both at sea and on land.

The researchers decided to study four fur seals. The animals spend weeks or months swimming in the ocean, but they come on land to mate and rear their young.

Oleg I. Lyamin, a neuroscientist who splits his time between U.C.L.A. and the Severtsov Institute of Ecology and Evolution in Moscow, implanted electrodes in the seals and strapped data recorders to their backs.

The fur seals lived in a pool where they could swim around or haul themselves onto a dry platform. After two days of recordings, the researchers took away the platform.

For up to two weeks, the seals could only swim in the pool. Then the researchers put the platform back, allowing the fur seals to doze out of the water again.

On the platform, the researchers found, the fur seals slept much as land mammals do. Their entire brains slipped into slow-wave sleep, interrupted from time to time by periods of R.E.M.

But when the seals had to sleep in the water, the brain patterns resembled those of dolphins. Only one hemisphere of their brain slept at a time. What’s more, the fur seals experienced almost no R.E.M. sleep.

“The R.E.M. sleep pretty much goes to zero and stays there as long as they’re in the water,” said Dr. Siegel.

When the seals got back on the platform, ordinary R.E.M. sleep returned. Their long spell of R.E.M.-free sleep did them no apparent harm, and they didn’t experience any R.E.M.-sleep rebound.

The results undermine the idea that R.E.M. sleep is essential to mammals, like food and water, Dr. Siegel said. In fact, the earlier studies on R.E.M. deprivation might not have been as compelling as they once seemed.

In those earlier studies, researchers kept animals from going into R.E.M. sleep by waking them up. “In some experiments, they wake up the animals a thousand times a day,” Dr. Siegel said.

The stress of being awakened over and over could have done the animals harm, rather than just the lack of R.E.M. sleep in particular.

A more telling clue about R.E.M. sleep can be found in human behavior, Dr. Siegel thinks. When people wake up on their own, they tend to move out of R.E.M. sleep and become alert. Those awakened from slow-wave sleep are groggy and disoriented.

Dr. Siegel and his colleagues propose that the brain cools during slow-wave sleep. To keep the brain from getting too cold, however, the brain periodically unleashes a torrent of activity. Oxygen-rich blood flows into the brain to fuel the activity, warming the brain in the process.

“It keeps the brain temperature within a functional limit by cycling on and off the same way your heater in your house might do at night,” Dr. Siegel said.

This explanation could also account for why dolphins don’t experience R.E.M. sleep — and why seals don’t as they swim.

These marine mammals have evolved a half-brain style of sleeping, perhaps as a way to remain alert enough to avoid predators and drowning.

Because part of the brain is always active, it’s always warm. As a result, it never triggers R.E.M. sleep. Only when fur seals return to land and switch to sleeping with their entire brains do the organs cool enough to flip the switch.

Niels C. Rattenborg, a sleep expert at the Max Planck Institute for Ornithology in Seewiesen, Germany, said the new study is “another compelling piece of evidence questioning the importance of R.E.M. sleep.”

He found Dr. Siegel’s brain-shiver theory intriguing. But he wanted to see it put to the test.

Neuroscientists have identified a clump of neurons in the brain stem as the switch that turns on R.E.M. sleep. Dr. Siegel’s theory predicts that in fur seals the brain stem stays warm at sea but cools on land.

“Until we have that test, it’s an interesting idea but remains unproven,” Dr. Rattenborg said.

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