Cold winter mornings are characterized by icy roads, dark skies, and millions of people struggling to make their way out of bed. But why exactly is it so universally difficult to get up and moving during the colder months of the year? A new study from Northwestern University has an answer, and all it took was a little help from some fruit flies.
Just like people, fruit flies are just a little bit slower to get going when it’s colder outside. After studying a group of these insects, researchers discovered a “thermometer circuit” within their antennae that send temperature information about the outside environment directly to their brains. So, via this circuit, seasonal temperatures and light conditions are detected and subsequently inhibit neurons within the fly brain that activate wakefulness.
Now, humans don’t have an antenna connected to their brain, but we’re just a little more complex than your average fruit fly. Our bodies are capable of picking up on temperature and subconsciously sending that information to the brain without an antenna.
“This helps explain why — for both flies and humans — it is so hard to wake up in the morning in winter,” says Marco Gallio, associate professor of neurobiology in the Weinberg College of Arts and Sciences, in a university release. “By studying behaviors in a fruit fly, we can better understand how and why temperature is so critical to regulating sleep.”
Interestingly, these temperature receptors in flies’ antennae only appear to activate when the weather drops below their “comfort zone” (roughly 77 degrees Fahrenheit). Once they had successfully located these receptors, researchers traced them back to their targeted areas within the brain. They discovered the temperature information is being sent to a small group of very specific neurons responsible for activity and sleep patterns.
Moreover, according to their observations, when a fly’s “cold circuit” is activated and sends temperature data to the brain, the neurons that would normally “turn on” in response to morning light are shut down completely.
Fruit flies are cold-blooded, and thus require very warm temperatures to survive and thrive. Humans, on the other hand, are mostly concerned with comfort when it comes to temperature. We’re all lucky enough to live in a time of indoor heating and air conditioning, so extremes on either end of the thermometer aren’t a big concern. Still, that doesn’t stop us from constantly adjusting the temperature in our homes and bedrooms.
Sleep is one of the main reasons humans are so sensitive to slight temperature fluctuations; both brain and core temperatures are strongly connected to one’s ability to fall and stay asleep. Additionally, it’s been understood for centuries that humans’ sleep patterns are influenced by seasonal changes in daylight and weather.
“Temperature sensing is one of the most fundamental sensory modalities,” Gallio says. “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.”
So, if we go with the researchers’ hypothesis that humans’ brains are receiving temperature information similarly to flies, that means our brains are essentially told by cold weather to stay asleep.
The brain cells that tell your brain to wake up every morning are given an urgent message on cold mornings: “go ahead and sleep in, it’s way too cold outside anyway.”
“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,” comments co-author Michael H. Alpert, a postdoctoral fellow in Gallio’s lab.
A lot of more research is ultimately needed before we can fully understand the relationship between temperature and wakefulness in humans. Still, just like a bear hibernating through the winter, cold weather seems to “instruct” our brains to hit the snooze button.
“It is crucial to study the brain in action,” co-author Dominic D. Frank, a former Ph.D. student in Gallio’s lab, concludes. “Our findings demonstrate the importance of functional studies for understanding how the brain governs behavior.”
The full study can be found here, published in Current Biology.