How Your Brain Fights Sleep Even When You're Exhausted

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iStock

Maybe you’ve got an exam in the morning, or there’s just one more episode left in this season. Whatever the reason, tonight you ignore your body’s demands and stay up instead. It’s an impressive feat, if you think about it—sleep is essential—and now scientists may be closer to understanding how we do it. They published a report on their findings in the journal Neuron.

There’s a little section of your brainstem called the dorsal raphe nucleus (DRN). This region is responsible for making serotonin and other brain chemicals.

Lead researcher Viviana Gradinaru of Caltech says previous studies have also suggested that the dorsal raphe nucleus plays a role in helping keep us awake.

"People who have damage in this part of their brain have been shown to experience excessive daytime sleepiness,” she said in a statement, “but there was not a good understanding of the exact role of these neurons in the sleep/wake cycle and whether they react to internal or external stimuli to influence arousal."

Within the dorsal raphe nucleus lies a little-understood group of dopamine cells called the dorsal raphe nucleus neurons (DRNDA).

Color image of dorsal raphe nucleus neurons
Dorsal raphe nucleus neurons responding to light (green) and chemical (red) signals.
Viviana Gradinaru

Gradinaru and her colleagues wanted to know if voluntary wakefulness had anything to do with dopamine activity within these cells. They started by studying mouse brains, which are similar to our own in many ways.

The researchers monitored the rodents’ DRNDA action while the mice were fed, met new potential mates, or experienced sudden unpleasant sensations—all experiences for which the mice would want or need to stay awake. Throughout the experiences, the mice’s DRNDA cells kept very busy, sending bursts of dopamine to other parts of the brain.

Next, the scientists tracked DRNDA cell activity as the mice slept and woke. They found that the cells seemed to sleep when the mice did, and revved up when the mice got up.

So far, the researchers knew that the sleeping mouse/sleeping neurons and waking mouse/waking neurons pairs existed, but they couldn’t tell if the neurons caused the waking or vice versa.

To find out, they engineered DRNDA cells that could be switched on and off by light. They then bred mice with these light-sensitive cells and let them sleep. As the mice snoozed, the researchers switched on the lights and their DRNDA cells using a technique called optogenetics. Sure enough, the mice woke up.

Shutting off DRNDA cells had the opposite effect: Mice with no DRNDA activity couldn’t keep their eyes open, even when faced with danger, loud noises, or the possibility of mating.

The authors note that their experiments included only mice, and that it’s too soon to draw conclusions about what this might mean for people.

“Further work is necessary to establish causation in humans,” Gradinaru said, “and to test the potential of the DRNDA as a therapeutic target for insomnia or oversleeping, and for sleep disturbances that accompany other psychiatric disorders such as depression, bipolar disorder, and schizophrenia."

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Winter is Coming: Why Some People Seem to Feel Colder Than Others

Work blanket? Check. Hot tea? Check. Writing gloves? Check.
Work blanket? Check. Hot tea? Check. Writing gloves? Check.
shironosov/iStock via Getty Images

For a few weeks a year, as winter turns into spring, or summer gives way to fall, people in heavy coats coexist with those in sandals and shorts. Similarly, in an office where the thermostat is set at 74°F, some workers will be comfortable in short sleeves, while others will be wearing sweaters and scarves.

Underlying this disagreement are the different ways people perceive cold—and scientists are still trying to understand them.  

Men, Women, and Metabolism

In work settings, men and women often have different opinions about the ideal temperature. A 2019 study found that women performed better in math and verbal tasks at temperatures between 70°F and 80°F, while men did better below 70°F. The researchers proposed that gender-mixed workplaces might boost productivity by setting the thermostat higher than the current norm (which the Occupational Safety and Health Administration suggests should be between 68°F and 76°F).  

The discrepancy has a known physical basis: Women tend to have lower resting metabolic rates than men, due to having smaller bodies and higher fat-to-muscle ratio. According to a 2015 study, indoor climate regulations are based on an “empirical thermal comfort model” developed in the 1960s with the male workers in mind, which may overestimate female metabolic rates by up to 35 percent. To compound the problem, men in business settings might wear suits year-round, while women tend to have more flexibility to wear skirts or sundresses when it's warm outside.

Culture and the Cold

Cultural factors are also involved. European visitors are habitually alarmed by the chilly temperatures in American movie theaters and department stores, while American tourists are flabbergasted at the lack of air conditioning in many European hotels, shops, and offices. The preferred temperature for American workspaces, 70°F, is too cold for Europeans that grew up without the icy blast of air conditioners, Michael Sivak, a transportation researcher formerly at the University of Michigan, told The Washington Post in 2015.

The effects of cultural change on the human ability to withstand extreme temperatures can be dramatic. In the 19th century, 22 percent of women on the Korean island of Jeju were breath-hold divers (haenyeo). Wearing thin cotton bathing suits, haenyeo dove nearly 100 feet to gather shellfish from the sea floor, holding their breath for more than three minutes in each dive. In winter, they stayed in 55°F-57°F water for up to an hour at the time, and then warmed up by the fire for three of four hours before jumping back in.

In the 1970s, haenyeo starting wearing protective wet suits. Studies conducted between the 1960s and the 1980s showed that their tolerance for cold diminished [PDF].

Blame Your Brain

Beyond the effects of cultural practice and body composition, scientists have started to identify the cognitive factors that influence our temperature perception. It turns out that what feels unpleasantly cold versus comfortably chill is partly in our own minds.

One example is the phenomenon described as “cold contagion.” A 2014 study asked participants to view videos of people immersing their hands in visibly warm or cold water. Observers not only rated the hands in cold water as cooler than those in hot water, but their own hands became cooler when watching the cold-water videos. There was no comparable effect for the warm water videos, however. The findings suggest that we may feel colder when surrounded by shivering people at the office than if we're there by ourselves, even when setting the thermostat at the same temperature in both cases.

Other studies highlight the psychological aspects of temperature perception. Experimental participants at the Institute of Biomedical Investigations in Barcelona, Spain, watched their arms become blue, red, or green by means of virtual reality, while the neuroscientist Maria Victoria Sanchez-Vives and her team applied heat to their actual wrists. As the temperature increased, participants felt pain earlier when their virtual skin turned red than when it turned blue or green.

Subjectivity in temperature perception has led to some creative treatments for burn patients. In the 1990s, Hunter Hoffman, David Patterson, and Sam Sharar of the University of Washington developed a virtual-reality game called SnowWorld, which allows patients in hospital burn units to experience virtual immersion in a frozen environment. Amazingly, playing SnowWorld counteracted pain during wound care more effectively than morphine did.

“The perception of temperature is influenced by expectations,” Sanchez-Vives tells Mental Floss. “Putting one’s hand inside a virtual oven is perceived as ‘hot,’ while sticking one’s hand into a virtual bucket filled with iced water is perceived as ‘cold,’ despite being at room temperature in each scenario.”

In other words, if you expect to feel cold walking into the office or out on the street, chances are that you will.