Why does heat make us sleepy? Fruit flies ‘programmed’ to take naps may hold the answer

A brain ‘thermometer’ offers some clues, reports Tom Batchelor

Many people enjoy a nap in hot weather (Alamy/PA)

The Spanish love a siesta, while in the UK a summer heatwave is the perfect excuse for an afternoon nap. But the reason hot weather makes people sleepy is not fully understood.

Now, however, neurobiologists at Northwestern University in the US state of Illinois have turned to fruit flies (Drosophila) for the answer.

Analysing the insects’ brain “thermometer” circuit, they found that neurons receiving information about heat are part of a wider system that regulates sleep.

When the so-called hot circuit – which runs parallel to the cold circuit – is active, when the temperatures climb, the target cells that promote midday sleep stay on longer.

The result for the fruit fly is an increase in sleep during the day, when the temperatures are highest. In essence, the fly is programmed to take a nap when the mercury rises – much like humans have done for centuries to avoid the worst of the midday heat.

“Changes in temperature have a strong effect on behaviour 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 at Northwestern University.

“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 centres remain largely unmapped.”

Mr Gallio’s paper, published in the journal Current Biology, is the first to identify “absolute heat” receptors in a fly’s head which respond to temperatures above 25C — the fly’s (and, biologically speaking, human’s) favourite temperature.

Drosophila melanogaster, or the common fruit fly, holds many secrets about biology (Getty)

Researchers were able to carry out their work thanks to a 10-year initiative that produced the first completed map of neural connections in an animal – a fly – called the connectome.

This computer system tells them all possible brain connections for 100,000 brain cells in the fly’s head, and provided the team with a detailed road map offering clues to the different effects of hot and cold temperatures on physiology and behaviour.

Mr Gallio said: “People may choose to take an afternoon nap on a hot day, and in some parts of the world this is a cultural norm, but what do you choose and what is programmed into you?

“Of course, it’s not culture in flies, so there actually might be a very strong underlying biological mechanism that is overlooked in humans.”

The researchers said their study had started the process of discovery about sleeping patterns but more work was needed to learn more about the effects of hot and cold weather.

“We identified one neuron that could be a site of integration for the effects of hot and cold temperatures on sleep and activity in Drosophila,” said Michael Alpert, the paper’s first author and a post-doctoral researcher in the Gallio lab.

“This would be the start of interesting follow-up studies.”