Why does the heat make us drowsy?

On hot summer days, around noon, when the temperature is the highest of the day, we often get lazy and this ends up leading to drowsiness. In some parts of the world, taking a nap in the midday time slot is a widely followed social custom and part of the local culture. There, during the hottest hours of the day, many commercial establishments are closed and many people sleep or at least relax. It turns out that biology, and not just culture, may be behind this.

Temperature affects in one way or another the entire range of typical human behavior, from how much we eat, to the sleep-wake cycle. It’s not uncommon to find it harder to sleep on a hot summer night and harder to get out of bed on cooler mornings. But about the link between the sensory neurons and those that control this cycle, there is still much that is not understood.

Some neurobiologists at Northwestern University in the United States have found some clues about what happens when the temperature of the environment. influences our level of sleepiness.

In the study carried out by Marco Gallio’s team, it was found that fruit flies are preprogrammed to take a nap in the middle of the day.

This study is the first to identify “absolute heat” receptors on the heads of flies. Those receptors respond to temperatures above 25 degrees Celsius, the favorite temperature of flies, which is also that of many humans. Not surprisingly, the favorite temperature of flies also coincides with that of many humans; the common fruit fly (Drosophila) has colonized almost the entire planet forming a close association with humans.

Flies are preprogrammed to take a midday nap. (Image: Gallio Lab/Northwestern University)

The researchers discovered that the neurons in the fly brain that receive information about heat are part of a larger system that regulates sleep. When the circuitry that reacts to heat is active, the cells that promote midday sleepiness remain activated longer. This results in increased drowsiness around noon, which keeps flies inactive in a corner rather than flying in full sun during the hottest part of the day.

The study is titled “A thermometer circuit for hot temperature adjusts Drosophila behavior to persistent heat.” And it has been published in the academic journal Current Biology. (Font: NCYT by Amazings)