Why Does Heat Make You Sleepy?



On the warmest summer days, you could find yourself nodding off in the midst of the day. In several regions of the world, it's usual to schedule "siestas" and close businesses at the warmest times of the day. In actuality, biology rather than just society might be to blame for this.

Temperature has an overall impact on human behavior, including eating, activity levels, and sleep-wake cycles. We can have problems going asleep in the summer, and we might take longer to wake up in the winter. However, little is known about the connection between sensory neurons and the neurons that regulate this cycle.

Northwestern University neurobiologists have found some clues as to what is occurring. Fruit flies are physically predisposed to taking naps in the middle of the day, according to a recent study that was just published in Current Biology. A comparable "thermometer" circuit for hot temperatures is investigated in the current work, which is a follow-up to their 2020 Biology article that identified a brain thermometer that is only functional in cold temperatures.

According to Marco Gallio, an associate professor of neuroscience at the Weinberg College of Arts and Sciences, "Changes in temperature have a strong impact on behavior in both humans and animals, and offer animals a cue that it is time to adapt to the changing seasons." The particular brain circuits that govern the relationship between temperature and sleep centers are still largely unmapped. The influence of temperature on sleep can be quite dramatic, with some species choosing to sleep off a whole season.

Fruit flies are a particularly useful model for examining important questions like "why do we sleep" and "what does sleep do for the brain," according to the study's lead author, Gallio, because they don't attempt to disrupt instinct in the same way that people do, for example, when they stay up all night studying. Additionally, they enable researchers to look into how environmental factors like temperature and light affect biological processes.

In fly heads, "absolute heat" receptors have now been discovered for the first time. These receptors react to temperatures over 77 degrees Fahrenheit, which is the fly's preferred temperature. It turns out that through establishing a close relationship with humans, the common laboratory fruit fly (Drosophila) has practically conquered the entire world. It should come as no surprise that its preferred temperature is the same as many humans.

Researchers discovered that brain neurons that receive information about heat are a component of the larger system that regulates sleep, which is exactly what they predicted based on the findings of their earlier work on cold temperature. The target cells that encourage midday sleep remain active for a longer period of time when the hot circuit, which runs parallel to the cold circuit, is active. This causes a rise in midday sleep, which deters flies from congregating at the hottest time of the day.

The discovery was made possible by a 10-year project that created the connectome, the first comprehensive map of brain connections in an animal (a fly). The connectome gives researchers access to a computer system that lists every potential connection between each of the approximately 100,000 brain cells in the fly. Even with this incredibly thorough road map, however, scientists still need to understand how information in the brain travels from point A to point B. This essay fills that gap.

Gallio claimed that "hot and cold temperatures can have extremely diverse impacts on physiology and behavior," which explains why there are different circuits for hot and cold temperatures. This division might possibly be a result of evolutionary processes influenced by Earth's yearly heat and cold cycles. For instance, based on this work, it is now possible to explore whether human sleep-related brain regions may be specifically targeted by a particular sensory circuit.

The next step is for Gallio's team to identify the shared targets of the cold and hot circuits in order to learn how each can affect sleep.

According to Michael Alpert, a post-doctoral researcher in the Gallio lab and the paper's first author, "We found one neuron that might serve as a point of integration for the impacts of hot and cold temperatures on sleep and activity in Drosophila." This would be the beginning of fascinating follow-up research.

In order to comprehend the implications of global warming, Gallio continued, the team is interested in examining the long-term effects of temperature on behavior and physiology, as well as how adaptive species are to change.                                     
In some parts of the world, taking a sleep in the middle of the day on a hot day is a cultural norm. But what do you pick, and what is ingrained in you? said Gallio. Naturally, it isn't culture in flies, so it's possible that humans are missing a really potent underlying biological process.

The National Institutes of Health provided funding for the study.

By NORTHWESTERN UNIVERSITY 

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