REM sleep may exist to warm the brain from the inside: ScienceAlert

Even if the contents of your dreams aren’t hot or steamy, slipping into rapid eye movement (REM) sleep can still warm you up from the inside, according to a new review.

In the wild, warm-blooded creatures with lower body temperatures tend to have longer periods of REM sleep; while those with higher body temperatures, such as birds, experience less REM sleep overall.

Neurologist and leading sleep scientist Jerome Siegel of the University of California, Los Angeles, argues that the association is remarkable and should be investigated further.

Siegel argues REM sleep could be a kind of “like a tremor for the brain” wThe hen’s brain and body temperatures drop too low during non-REM sleep.

During REM the brain becomes very active, which increases the temperature of the organ. Also, REM sleep almost always follows non-REM sleep, which is when the brain and body are less active and cooler.

“REM sleep could be thought of as a thermostatically controlled brain warming mechanism, which is triggered by temperature reduction linked to reduced metabolism and decreased energy expenditure in non-REM sleep,” writes Siegel.

“Then REM sleep ends after the amount of REM needed to raise the brain temperature to near the body’s waking temperature has occurred.”

In fact, this could be why some animals show fluctuations in sleep duration from one season to the next. The most extreme example of this is hibernation, but even non-hibernating animals like arctic reindeer sleep 43 percent more in winter than in summer. Humans in hunter-gatherer societies also sleep about an hour longer during the winter months.

Could REM sleep help protect animals’ brains from the cold while allowing them crucial rest time?

Siegel thinks it’s entirely possible, especially since other hypotheses about REM sleep have been shown to be flawed.

Some scientists, for example, have suggested that non-REM sleep helps remove toxins from the brain, while REM sleep helps improve memory and learning, possibly by trimming neural connections to make the brain more efficient.

But here’s the confusing thing: In almost all mammals, non-REM sleep is followed by REM sleep, which is a state of very high brain activity, similar to wakefulness. This would mean that right after the toxins and synapses in the brain are cleared, they would simply be created again.

Furthermore, there is no obvious relationship between REM sleep duration and cognitive power, suggesting that its potential role in learning may be overstated. Platypuses, for example, experience up to 8 hours of REM sleep per night, more than any other animal, including humans. It’s hard to argue that the platypus may need this stage of sleep to gain extra brain efficiency.

On the other hand, this strange creature is a monotreme, a kind of middle ground between a cold-blooded and a warm-blooded animal. According to Siegel’s hypothesis, this means that the platypus would need more REM sleep to maintain a functional brain temperature while falling asleep.

Therefore, REM sleep may have initially evolved as a way for endotherms to keep the brain warm and functional in case it is awakened by a threat.

As mammals that show no signs of REM sleep, dolphins may be an exception that proves the rule. These abnormalities are thought to be involved in unihemispheric sleep, where only one side of the brain falls asleep at a time. In these rare cases, brain temperature may not be so easily influenced by sleep because a “space heater” is still operating in one part of the “room,” reducing the need for episodic rewarming.

Migratory birds, on the other hand, show some signs of REM sleep, although they also engage in unihemispheric sleep. But because this stage of sleep involves both sides of the brain, these types of birds only enter REM for very short periods of time. As you can imagine, flying with an inactive brain could be very dangerous.

Siegel believes that the uni-hemispheric exception can be tested more in fur seals, which sleep with both sides of their brains on land and only one side in water.

The idea that REM sleep keeps the engine going for animals like us is complicated by the ongoing debate around it REM sleep in cold-blooded reptiles, that although it has not yet been confirmed, it cannot be ruled out either.

Sleeping to conserve energy is crucial, but animals need to make sure they can still wake up to a threat. If Siegel is right, REM sleep could be a hot new solution to an old conundrum.

The study was published in The Lancet.

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