These Summer Heatwaves Really Do Make Us Sleepy, New Research Shows

According to a study conducted by researchers at Northwestern University, napping when it's hot outside may be more biological than cultural.

woman sleeping in bed comfortably
Photo: Maca and Naca / Getty Images

It's no secret that the heat has been unbearable lately—a simple trip to the grocery store or coffee shop (for iced beverages only!) feels like a chore as temperatures climb. But the heat doesn't just make you uncomfortable: It also makes you extra sleepy. According to a new study conducted by researchers at Northwestern University, there is a reason for this, and it's likely biological.

The research, which was published in the journal Current Biology, found the fruit flies are pre-programmed to take a nap in the middle of the day. According to ScienceDaily, the neurobiologists used fruit flies to find out why heat makes humans sleepy because they don't attempt to disrupt instinct in the same way we do. Additionally, they allow researchers to study the influence of external cues like light and temperature on their cellular pathways.

"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, lead study author, and associate professor of neurobiology in the Weinberg College of Arts and Sciences. "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."

Scientists were able to identify how flies respond to heat due to a 10-year initiative that produced the first complete map of neural connections in a fly. Thanks to this, they have access to a computer system that tells the researchers every possible brain connection in a fly's 100,000 brain cells.

They found that the brain neurons receiving information about heat are more general than the system that regulates sleep. ScienceDaily reports that when the hot circuit is active, the target cells that encourage an afternoon nap stay on longer—which results in an increase in midday sleep that prevents flies from enduring the hottest part of the day. These receptors typically kick in when temperatures go above 77 degrees—a fly and human's favorite temperature.

With their findings, the team now hopes to uncover the common targets of the cold and hot circuit to find how each impacts sleep. Additionally, the team is also interested in observing the longterm effects of temperature on behavior. "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?" says Gallio. "Of course, it's not culture in flies, so there actually might be a very strong underlying biological mechanism that is overlooked in humans."

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