We've all had the experience of sleeping at a relative's house where the heat is stifling or staying in a hotel room whose air conditioner just won't turn off. In both cases, it's difficult to get a good night of sleep. Neuroscientists in Switzerland recently found a reason for that: They identified a population of "Goldilocks neurons" that increase REM sleep when the room temperature is just right.
I Don't Sleep, I Dream
There are two different kinds of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. When you first fall asleep, you enter NREM sleep, which includes a few different stages. In stage one, your heartbeat, breathing, and eye movements slow and your muscles relax as you transition from wakefulness to sleep. In stage two, your eye movements stop completely and your body temperature drops. Stage three is the deepest period of sleep — this is when you get the rest your tired mind and body crave. Your breathing, heartbeat, and brain waves slow to the lowest possible levels. At this point, it'd be really difficult for someone to wake you up.
After those three stages of NREM sleep comes REM sleep, named for the quick darting movements that your eyes make beneath your closed eyelids. This stage, also known as dream sleep, occurs about 90 minutes after you fall asleep when your breathing, heartbeat, and brain activity increase to reach near-waking levels. Your muscles, on the other hand, become paralyzed to keep you from acting out your dreams.
As the night goes on, you cycle through stages of NREM and REM sleep, with longer, deeper REM stages taking place in the early morning. While all of these sleep stages are important, REM seems to be particularly essential for consolidating the memories and lessons you've learned throughout the day. Studies of lab rats trained on a maze task, for example, had a drop in performance when they were deprived of REM sleep.
Not Too Hot, Not Too Cold
There's a lot of brain activity that takes place during REM sleep, but one thing your brain can't do at that point is regulate your body temperature.
"This loss of thermoregulation in REM sleep is one of the most peculiar aspects of sleep, particularly since we have finely-tuned mechanisms that control our body temperature while awake or in non-REM sleep," said Markus Schmidt, a neuroscientist at the University of Bern and senior author of the study, in a press release.
Those mechanisms include things like panting, sweating, shivering, and getting goosebumps. All of those things require a lot of energy, so in order to have the energy to power the brain functions required of REM sleep, your body pumps the brakes on thermoregulation when you're sleeping. Of course, if the room gets too hot or cold and your body doesn't react, you're in trouble, so there must be a way for the brain to shuttle energy back to thermoregulation when you need it. Schmidt hypothesized that there are mechanisms in the brain that regulate the amount of REM sleep you get depending on the room temperature, like increasing REM sleep when the room is comfortable and sacrificing REM sleep when it's too hot or cold.
When Schmidt and his team examined the brains of mice, their hunch was confirmed: There's a specific population of neurons in the hypothalamus that does just that. These neurons, called melanin-concentrating hormone (MCH) neurons, were found to increase REM sleep when the room temperature is at the high end of one's comfort zone. Mice that were genetically engineered to lack MCH receptors didn't increase REM sleep at warm temperatures, essentially rendering them temperature-blind in terms of sleep regulation.
This study, published in Current Biology, not only confirms the link between REM sleep and thermoregulation but also has implications for sleep medicine. It shows that the amount of REM sleep you get — which, again, is crucial for consolidating memories — is directly dependent on your immediate environment.
But what temperature is best for these neurons? Schmidt's team found that temperatures at the high end of the body's thermoneutral zone were best. That zone varies a lot from person to person, which is why experts generally recommend setting the thermostat to whatever temperature you find comfortable. The next time you get ready for bed, think Goldilocks and make sure the temperature is just right.