Fruit flies reveal why we sleep more in cold weather

Why is it harder to wake up on a cold morning? A new study from Northwestern University has tapped into the minds of fruit flies to find out how cold weather affects our sleep.

While it is known that temperature has a profound influence on the behavior of people and animals, the connection between neurons that control the sleep-wake cycle and those that respond to external stimuli is not clearly understood. 

In a study focused on fruit flies, researchers have identified a sensory circuit that relays information about cold temperatures to a higher brain center. 

According to the experts, through this sensory circuit, cold and dark conditions can inhibit the brain cells that promote wakefulness and activity. The neural network was found to have a stronger effect in the morning. 

Study lead author Marco Gallio is an associate professor of Neurobiology in Northwestern’s Weinberg College of Arts and Sciences. 

“This helps explains why – for both flies and humans – it is so hard to wake up in the morning in winter,” said Professor Gallio. “By studying behaviors in a fruit fly, we can better understand how and why temperature is so critical to regulating sleep.”

The study is the first to describe “absolute cold” receptors in the fruit fly’s antenna. The receptors respond exclusively to temperatures that are below the fly’s comfort threshold of approximately 77 degrees Fahrenheit. 

The researchers traced the neurons to their targets in the brain, which turned out to be a small group of cells in a region that controls circadian rhythm.

When the cold circuit was stimulated, target cells that are normally activated by morning light were shut down. 

The circadian rhythm of the fruit fly is a daily 24-hour cycle of rest and activity, just like that of humans. 

The detection of external temperature is critical for survival, especially among tiny fruit flies, but humans also seek optimal core temperatures simply for comfort or to enhance sleep. 

“Temperature sensing is one of the most fundamental sensory modalities,” said Professor Gallio. “The principles we are finding in the fly brain – the logic and organization – may be the same all the way to humans. Whether fly or human, the sensory systems have to solve the same problems, so they often do it in the same ways.”

Study co-first author Michael H. Alpert is a postdoctoral researcher in Professor Gallio’s lab.

“The ramifications of impaired sleep are numerous – fatigue, reduced concentration, poor learning and alteration of a myriad of health parameters – yet we still do not fully understand how sleep is produced and regulated within the brain and how changes in external conditions may impact sleep drive and quality,” said Alpert.

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer