"Brain thermometer" explains why heat makes us feel sleepy

Neurobiologists at Northwestern University may have just solved the mystery of why heat makes us sleepy. In a new study focused on fruit flies, the researchers identified a “brain thermometer” that promotes sleep during the hottest hours of the day. 

“Small poikilotherms such as the fruit fly Drosophila depend on absolute temperature measurements to identify external conditions that are above (hot) or below (cold) their preferred range and to react accordingly. Hot and cold temperatures have a different impact on fly activity and sleep, but the circuits and mechanisms that adjust behavior to specific thermal conditions are not well understood,” wrote the researchers.

In 2020, the Northwestern team identified a brain thermometer that is only active in cold weather. The new study describes a similar brain circuit that is activated by hot temperatures.

“Changes in temperature have a strong effect on behavior in both humans and animals, and offer animals a cue that is time to adapt to the changing seasons,” said Professor Marco Gallio, who led the study.

“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 centers remain largely unmapped.”

Professor Gallio explained that fruit flies are a particularly good model to study big questions, such as why we need sleep, because they do not attempt to disrupt instinct in the same way humans do – like when we pull all-nighters.

The study is the first to identify “absolute heat” receptors in the fly head. According to the researchers, these heat receptors respond to temperatures above about 77 degrees Fahrenheit – the fly’s optimal temperature. 

Professor Gallio said it makes sense that there are different brain circuits for hot and cold temperatures because they can have quite different effects on physiology and behavior. Next, the team hopes to identify the common targets of the cold and hot circuit in an effort to determine how each can influence sleep.

“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 study first author Michael Alpert. “This would be the start of interesting follow-up studies.”

The team is interested in looking at the long-term effects of temperature on behavior and physiology to understand the impact of global warming, looking at how adaptable species are to change, noted Professor Gallio.

“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 study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer