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 nature, warm-blooded creatures with lower body temperatures tend to have longer periods of REM sleep; while people with higher body temperatures, such as birds, experience less REM sleep in general.
Neurologist and leading sleep scientist Jerome Siegel, of the University of California Los Angeles, says the association is remarkable and needs further investigation.
Siegel argues REM sleep can be a kind of “brain trembling”Brain and body temperatures drop too low during non-REM sleep.
During REM, the brain becomes very active, causing the temperature of the organ to rise. In addition, REM sleep almost always follows non-REM sleep, which is when the brain and body are least active and cold.
“REM sleep can be thought of as a thermostatically controlled brain heating mechanism, which is activated by the temperature reduction associated with the decreased metabolism and decrease in energy expenditure in non-REM sleep,” Siegel writes.
“Then REM sleep ends after the amount of REM needed to raise brain temperature to close to the body’s waking temperature.”
That may be why some animals show seasonal fluctuations in sleep duration. The most extreme example of this is hibernation, but even animals that don’t hibernate, such as arctic reindeer, sleep 43 percent more in winter than in summer. People in hunter-gatherer communities also sleep about an hour longer in the winter months.
Can REM sleep help protect animals’ brains from the cold, while still giving them crucial rest time?
Siegel thinks it’s entirely possible, especially since other hypotheses surrounding REM sleep have turned out to be imperfect.
For example, some scientists have suggested that non-REM sleep helps clear toxins from the brain, while REM sleep helps improve memory and learning, possibly by cutting back neural connections to make the brain more efficient.
But here’s the confusing part: In almost all mammals, non-REM sleep is followed by REM sleep, a state of very high brain activity, similar to waking up. This would mean that immediately after toxins and synapses in the brain are cleared, they would simply be recreated.
In addition, there is no clear relationship between REM sleep duration and cognitive power, suggesting that its potential role in learning may be overestimated. Platypus, for example, experience up to 8 hours of REM sleep per night — longer than any other animal, including humans. It’s hard to argue that the platypus needs this sleep phase for extra brain efficiency.
On the other hand, this bizarre creature is a monotreme – a sort of middle ground between a cold- and warm-blooded animal. According to Siegel’s hypothesis, this means that the platypus would need more REM sleep to instead maintain a functional brain temperature while it dozes.
REM sleep may therefore have evolved initially as a way for endotherms to keep their brains warm and functional in case they are awakened by a threat.
As mammals that show no signs of REM sleep, dolphins may be an exception that proves the rule. These anomalies are thought to participate in uni-hemispheric sleep, where only one side of the brain falls asleep at a time. In these exceptional cases, the brain’s temperature cannot be affected as easily by sleep as a ‘space heater’ is still operating in part of the ‘room’, reducing the need for episodic warm-ups.
Migratory birds, on the other hand, do show some signs of REM sleep, despite also participating in uni-hemispheric sleep. But because both hemispheres of the brain are involved in this sleep phase, these types of birds only get into REM for a very short time. As you can imagine, flying with an inactive brain can be very dangerous.
Siegel thinks the uni-hemispheric exception could be further tested in fur seals, which sleep on both sides of their brains on land and only one side in water.
The idea of REM sleep to keep the engine running for animals like us is complicated by the ongoing debate surrounding REM sleep in cold-blooded reptiles, which, although not yet confirmed, cannot yet be ruled out.
Sleeping to conserve energy is crucial, but animals must ensure that they can still wake up to a threat. If Siegel is right, REM sleep could be a new solution to an age-old conundrum.
The study is published in The Lancet.