Why does heat make us sleepy? – Science and future

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Une femme se reposant sur un hamac

Everyone will have noticed that heat often affects appetite, activity and biological rhythm. And the human being is not the only one interested. Researchers at Northwestern University investigated the effects of outdoor temperature on the brains of fruit flies (Drosophila melanogaster). Their big discovery: neurons that receive information about temperature, cold or hot, are part of the system that regulates sleep, according to the study published in Current Biology. By 2020, they had identified a brain thermometer active only in cold weather. Promotes morning sleep and reduces the time of afternoon naps. On this occasion, scientists have highlighted a circuit parallel to the previous one but activated by warm temperatures. “We already knew that high temperatures disturb sleep at night, but we have just discovered that heat also causes longer naps during the day,” summarizes Marco Gallio, first author of the study, for Sciences et Avenir. More than 4 hours of nap The fly is a so-called “crepuscular” animal, meaning that it is most active at dawn and dusk. The typical Drosophila day is therefore marked by two naps: one in the morning and one in the afternoon. At a temperature of 25°C, ideal for the fly, it rests for 1 hour in the morning, and 2.5 hours in the middle of the day. But below 30°C, the duration of their sleep doubles! But why? It would be a protection against high temperatures: animals feel less heat when they sleep. “The effect of temperature on sleep can be extreme, such as animals hibernating for an entire season, but the specific brain circuits governing the interaction between temperature and sleep centers remained largely unknown.” Marco Gallio points out. Everyone will have noticed that heat often affects appetite, activity and biological rhythm. And the human being is not the only one interested. Researchers at Northwestern University investigated the effects of outdoor temperature on the brains of fruit flies (Drosophila melanogaster). Their big discovery: neurons that receive information about temperature, cold or hot, are part of the system that regulates sleep, according to the study published in Current Biology. By 2020, they had identified a brain thermometer active only in cold weather. Promotes morning sleep and reduces the time of afternoon naps. On this occasion, scientists have highlighted a circuit parallel to the previous one but activated by warm temperatures. “We already knew that high temperatures disturb sleep at night, but we have just discovered that heat also causes longer naps during the day,” summarizes Marco Gallio, first author of the study, for Sciences et Avenir. More than 4 hours of nap The fly is a so-called “crepuscular” animal, meaning that it is most active at dawn and dusk. The typical Drosophila day is therefore marked by two naps: one in the morning and one in the afternoon. At a temperature of 25°C, ideal for the fly, it rests for 1 hour in the morning, and 2.5 hours in the middle of the day. But below 30°C, the duration of their sleep doubles! But why? It would be a protection against high temperatures: animals feel less heat when they sleep. “The effect of temperature on sleep can be extreme, such as animals hibernating for an entire season, but the specific brain circuits governing the interaction between temperature and sleep centers remained largely unknown.” Marco Gallio points out. The arista, as the upper part of the antennae is called, is the main sensory sensor of Drosophila. Each of the two ridges is formed by 3 sensilla (sensory organ carried by the integument of insects) that have thermal receptors: one for heat and one for cold. Heat receptors communicate with projection neurons called “TPNs”. These send stimuli to the posterior lateral neurons (PLN). These PNLs signal sleep regulatory cells. There are other cells that stimulate the posterior lateral neurons. These are called “anterior cells” or “AC” (nerve cells located in front of the head capsule). The originality of the above cells lies in their activation condition: only when the outside temperature exceeds 25°C! These are real built-in thermometers. In addition to their sensory role, they are hyperconnected cells! They send axons (an extension of the nerve cell that conducts the electrical signal) to a very large part of the fly’s brain, even inside it. Once activated, CAs send stimuli to posterior lateral neurons that interact with sleep regulatory cells. These two mechanisms thus promote sleep during the hottest hours of the day. The connectome: an essential tool This work could not have been carried out without the prior realization of the connectome. This is a complete map of the fly’s neural connections. Researchers in a previous study took no less than 12 years to map his brain, which was no bigger than a poppy seed. The fly’s 100,000 brain cells are represented, as well as all the possible connections between them. It is thanks to this detailed plan that the researchers were able to establish the 2 parallel circuits activated according to the temperature. “The human brain has a set of neurons that control circadian rhythms (alternate wakefulness/sleep),” explains Marco Gallio. According to the researcher, it is likely that these neurons are influenced in one way or another by temperature, just like those of the fly. Today, the team of scientists is particularly interested in the long-term effects of heat on behavior and physiology to understand the impact of climate change and the possibility of adaptation in certain species.
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