EVANSTON, Ill. — Is an afternoon “siesta” simply a cultural tradition, or is there something biological behind napping in the midday heat? A new study finds that there appears to be a “switch” in the brain that makes people want to sleep when the weather reaches a certain temperature.
It wouldn’t surprise many people to catch someone dozing off on a hot summer day. In certain parts of the world, businesses actually shut down during the warmest parts of the day, as people go home for a meal and a nap.
This isn’t the first study to examine the link between changes in temperature and sleep-wake cycles. Researchers have found that humans typically have a harder time getting quality sleep when it’s too hot, while others have discovered that people often have a hard time getting out of bed on cold mornings. However, the link between sensory neurons which feel these temperature shifts and neurons that control our sleep cycles has been unclear.
Your brain has its own ‘thermometer’
Now, a team at Northwestern University have found clues about what’s going on in the brain that makes people want to nap. In an experiment using fruit flies, study authors found that the insects are actually “pre-programmed” to take a nap in the midday heat. The findings follow up on a 2020 report that discovered a “brain thermometer” that activates in cold weather. The new study now reveals that the brain also has a thermometer for hot days as well.
“Changes in temperature have a strong effect on behavior in both humans and animals, and offer animals a cue that is time to adapt to the changing seasons,” says Marco Gallio, associate professor of neurobiology in the Weinberg College of Arts and Sciences, in a university release. “The effect of temperature on sleep can be quite extreme, with some animals deciding to sleep off an entire season — think of a hibernating bear — but the specific brain circuits that mediate the interaction between temperature and sleep centers remain largely unmapped.”
Gallio explains that fruit flies are particularly helpful test subjects when it comes to answering questions like “why do we sleep,” and “what does sleep do for the brain.” The reason for this is fruit flies don’t try to disrupt instinctive behaviors like humans do — by pulling all-nighters for work, for example. The flies also allow researchers to study how factors like light and climate affect their cellular pathways.
77 degrees Fahrenheit is the ‘sleep’ spot
Results show that a fly’s brain has an “absolute heat” receptor that responds to temperatures above 77 degrees Fahrenheit. This was the favorite temperature of the Drosophila, the common lab fruit fly found nearly all over the world where there are humans present. Interestingly, the team says this is also the favorite temperature for many humans as well.
Study authors also discovered that, just like in cold weather, brain neurons receiving information about hot weather are part of a larger biological system which regulates sleep. Moreover, this “hot circuit” targets cells that promote sleep during the middle of the day. In flies, this causes them to stay asleep during the hottest parts of the afternoon.
“People may choose to take an afternoon nap on a hot day, and in some parts of the world this is a cultural norm, but what do you choose and what is programmed into you?” Gallio says. “Of course, it’s not culture in flies, so there actually might be a very strong underlying biological mechanism that is overlooked in humans.”
The study is published in the journal Current Biology.