It's getting hot in here: Why do we get sleepy in the summer?- study

New research, the first of its kind, found that fruit flies are pre-programmed to nap during the day in heat, meaning the human nap may not just be cultural but also biological.

 Summer Camp Kimama Half Moon (photo credit: Camp Kimama)
Summer Camp Kimama Half Moon
(photo credit: Camp Kimama)

Find yourself napping more during the sweltering summer months?

According to a new study conducted by researchers at Northwestern University, there is a reason for your siesta, and it's likely biological. The research, published Wednesday in the journal Current Biology, discovered that fruit flies are pre-programmed to take a nap in the middle of the day.

"Hot and cold temperatures can have quite different effects on physiology and behavior."

Marco Gallio

The findings were a follow-up to a paper published in 2020 which identified a brain thermometer only active in cold weather, the new paper is the first to explore  a related “thermometer” circuit for hot temperatures.

 US Army soldiers sleep during a rare opportunity for a nap (credit: FLICKR) US Army soldiers sleep during a rare opportunity for a nap (credit: FLICKR)

“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,” said Marco Gallio, associate professor of neurobiology  and study leader. “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.”

But what do fruit flies have to do with human biology?

Gallio explained that fruit flies are an ideal model to study significant questions like “why do we sleep,” and “what does sleep do for the brain” because they don’t attempt to disrupt instinct in the same way humans do when we pull all-nighters, for example. Furthermore, they allow researchers to study the influence of external cues like light and temperature on cellular pathways.

"Hot and cold temperatures can have quite different effects on physiology and behavior,” Gallio continued, adding that this separation may also reflect evolutionary processes based on heat and cold cycles of the Earth. 

The team next hopes to uncover the typical targets of the cold and hot circuit to find how each impacts sleep. Additionally, the researchers expressed interest 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?" Gallio said. "Of course, it's not culture in flies, so there actually might be a very strong underlying biological mechanism that is overlooked in humans."