Why heat makes you tired and sleepy, according to science

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The reason you feel tired in the middle of the day may have to do with the weather, according to research. Crispin the Valiant/Getty Images
  • A recent study suggests that the desire for a mid-afternoon nap may be biologically determined and affected by outdoor temperatures.
  • Northwestern University scientists have found that the fruit fly is “pre-programmed” to rest in the middle of the day.
  • They are the first to find “absolute heat receptors” in the fly’s brain that respond to temperatures above 77 degrees Fahrenheit.

Outside temperatures can affect our energy, our emotions and the quality of our sleep. Scientists are still studying how climate change affects human behavior.

Neurobiologists at Northwestern University in Evanston, Illinois, may have discovered genetic underpinnings influencing the body’s adaptations to climate.

Their study recently published in the journal Current biology found a “distinct thermometer circuit” in the fruit fly brain triggered by high temperatures. It follows a paper 2020 which identified a “cold thermometer” circuit.

Lead author Marco Gallio, Ph.D., an associate professor of neurobiology at Northwestern University’s Weinberg College of Arts and Sciences, said Medical News Today:

“People may choose to take an afternoon nap on a hot day, and in some parts of the world it’s a cultural norm, but what do you choose and what is programmed into you? Of course, it’s not culture in flies, so there could be a very powerful underlying biological mechanism that’s overlooked in humans.

Medical News Today discussed this research with Dr. Gallio and asked him why he chose to examine the fruit fly (Drosophila).

The professor mentioned that sleep is universal throughout the animal kingdom. He also shared that 60% of insect genes are the same as humans.

The housefly appears all over the world due to its close association with people. His favorite temperature – 77 degrees Fahrenheit – is also close to that of many humans.

Dr Gallio said fruit flies are gaining traction in research because they exhibit a range of complex behaviors like humans. Yet “they are doing all of this in a brain that is only made up of 100,000 brain cells.”

On the other hand, the human brain contains about 86 billion brain cells.

In his article, “Ode to the Fruit Fly: Crucial Small Lab Topic for Basic Research,” Dr. Gallio wrote that our related anatomy and physiology make flies ideal for designing experiments “important to both animals and humans.” humans “.

The Northwestern University study builds on a 10-year project that produced the connectome, the first comprehensive map of neural pathways in an animal.

The connectome allowed researchers to analyze all possible neural connections for each Drosophila brain cell. The current study helped them observe how information in the brain moves from one point to another.

The antenna as a thermometer

The fly’s antenna has three organs called sensilla, each containing a heat-activated neuron and a cold-activated neuron.

The fly’s head also contains anterior cell (AC) neurons that respond to heat and cold. This research is the first to identify these “absolute heat receptors”.

During this study, Dr. Gallio and his colleagues noticed that heat-sensitive AC neurons are part of a larger network that controls sleep.

When the hot circuit was activated by temperatures above 77 degrees Fahrenheit, the cells triggering the noon sleep stay lit longer. This leads to longer sleep at midday, helping flies avoid movement during the hottest part of the day.

Matthew Walker, Ph.D., who was not involved in this study, is an author and professor of neuroscience and psychology at the University of California, Berkeley. Dr. Walker’s research focuses on sleep and human health.

In a 2019 podcast, Dr. Walker said, “Temperature is as powerful a trigger for sleep organization and depth as light. In order for you to fall asleep and stay asleep, your body must lower its core temperature by about one degree Celsius or about two degrees Fahrenheit. This is the reason why it will always be easier for you to fall asleep in a room that is too cold than too warm, because the room that is too cold at least takes you in the right thermal direction for a good sleep. […]”

The sleep expert also discussed studies from hunter-gatherer tribes that indicate the influence of temperature changes on sleep behavior. The way of life in these pre-industrial societies remained constant for thousands of years.

Dr Walker said these people don’t usually retire for the night immediately after sunset. On the contrary, the tribes go to bed several hours later when the ambient temperature drops.

Dr Walker commented: ‘It seems to be a thermal trigger for them to fall asleep and fall asleep. And […] they usually wake up 15 to 20 minutes before dawn. It is therefore not the light that seems to be necessarily the trigger for awakening. It’s actually the increase in temperature, and it’s about the circadian rhythm. So what drives us into our natural sleep rhythms is both temperature and light.

Jade Wu, Ph.D., a sleep psychologist, researcher, and lecturer who was not involved in the study, said DTM that she was curious if the results of the study could be extended to humans.

“We know that when it’s too hot, humans actually have a harder time sleeping, which seems to be the opposite of what happened with the flies in this study,” Dr Wu said.

Dr. Gallio agreed that humans tend to sleep better when it’s cold.

Dr. Gallio stressed that his work aims to uncover the “basic principles” that explain why we sleep and how temperature affects behavior.

“We don’t know much about these principles, but we should spend [more] Money on [learning about] these same principles before trying to [focus on] applied side [of research],” he said.

Michael Alpert, first author and post-doctoral researcher with Dr. Gallio, added: “We have identified a neuron that may be a site of integration of the effects of hot and cold temperatures on sleep and activity in Drosophila. This would be the start of interesting follow-up studies.

Dr. Gallio also said DTM that he hopes his work might inspire others to pursue further research, eventually human enquiries.

For example, this research opens the door to determining specific sensory circuits for brain regions for sleep in humans.

The professor said his team also wanted to examine the effects of climate change on behavior and physiology.

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