Why heat makes us sleepy: Fruit fly study reveals a thermometer brain circuit promoting midday siesta on hot days

On the hottest summer days, you may find yourself dozing off in the middle of the day. In some parts of the world, it’s a cultural norm to schedule “siestas” and shutter businesses during the warmest hours of the day. As it turns out, biology, not just culture, may be behind this.

Temperature affects the span of human behavior, from eating and activity levels to sleep-wake cycles. We may have a harder time sleeping in the summer and be slow to get out of bed on colder mornings. But the link between sensory neurons and neurons that control this cycle are not understood completely.

Northwestern University neurobiologists have found a few clues about what’s happening. In a new study, published today (Aug. 17) in the journal Current Biology, researchers found that fruit flies are pre-programmed to take a nap in the middle of the day. A follow-up to their 2020 Biology paper that identified a brain thermometer only active in cold weather, the new paper explores a similar “thermometer” circuit for hot temperatures.

“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 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.”

Gallio led the study and said fruit flies are a particularly good model to study big 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. They also allow researchers to study the influence of external cues like light and temperature on cellular pathways.

Cells that stay on longer

The paper is the first to identify “absolute heat” receptors in fly head, which respond to temperatures above about 77 degrees Fahrenheit — the fly’s favorite temperature. As it turns out, the common laboratory fruit fly (Drosophila) has colonized nearly the entire planet by forming a close association with humans. Not surprisingly, its favorite temperature also matches that of many humans.

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