Anyone who is out and about in Spain in the summer will have noticed: between 2pm and 4pm or often even 5pm nothing happens in some quarters, the doors remain closed. It doesn’t open again until later – but then also a little longer, because the actual evening doesn’t start until around 9 p.m. During closing time, the Spaniards take their traditional siesta, a nap after late lunch, which is also due to the particularly intense heat at this time, when almost nobody wants to go out voluntarily.
But where does the desire for a siesta come from, why do we get sleepy when it’s hot? A research group from Northwestern University in the US state of Illinois has now shown that this enduring tradition, which is enticing for residents of more northern climes, could have biological reasons. In animal experiments with Drosophila, the fruit fly, the scientists found that their brains have a kind of thermometer circuit that makes them tired when they are very hot. The siesta thus has an interesting neurological background, which seems to be effective even in organisms that are much more primitive than humans.
Fly connectome helps
Because the experiment showed that Drosophila also tends to take a nap in the middle of the day on warm days. This behavior is programmed, as a look into the simulated connectome that now exists for the fly’s 100,000 brain cells shows. The temperature sensors are apparently divided into warm and cold. The researchers had already illuminated the brain thermometer for the cold two years ago. Among other things, it could be partly responsible for the fact that some animals hibernate.
The ideal temperature for fruit flies is 25 degrees Celsius. If it gets warmer, their “absolute heat receptors” react and the animals begin to adapt their activity behavior. “Changes in temperature have powerful effects on behavior in both humans and animals,” says Marco Gallio, associate professor of neurobiology at Weinberg College for Arts and Sciences and co-author of the study. The temperature can also have “extreme effects” on sleep, up to and including hibernation. The specific areas of the brain that are responsible for these tasks now need to be mapped. A nerve cell that appears to integrate hot and cold temperatures to affect sleep has already been identified in Drosophila. It could become a starting point for further studies and thus provide further indications of the human need for an afternoon nap or a rest phase on warm days.
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