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Some scientists believe that temperature may play just as important a role as light in regulating the human circadian rhythm.

Anthropologist Gandhi Yetish still remembers a bedtime ritual from his childhood in New Jersey. During the cold winter months, he would open a window and let the chilly air cool his face. Even as he worried about his mother's complaints over the heating bill, he found that the cool air quickly made him feel sleepy.

A growing body of evidence suggests that Yetish's experience may have been onto something. Temperature can influence everything from how deeply we sleep to how refreshed we feel when we wake up. Some scientists go a step further, arguing that temperature may be just as important as light in determining when we fall asleep.

Here's what we know about the relationship between temperature and sleep—and what scientists have yet to figure out.

The Warmth of the Sun

For years, Yetish assumed that preferring a cool sleeping environment was simply a matter of personal taste. But his perspective began to change when he spent time with the Tsimane, an Indigenous group in the Bolivian Amazon, while studying sleep patterns in nonindustrial societies.

Like many scientists, Yetish had long accepted the conventional view that light exposure was the most important signal regulating sleep. But the Tsimane presented a puzzle. They had no electricity for artificial lighting and spent much of their day outdoors, yet they typically went to bed several hours after sunset and woke up before sunrise.

Yetish and his American colleagues later studied two other Indigenous groups: the Hadza of northern Tanzania and the San people of southern Africa. They found that the timing of these groups' sleep did not consistently align with the cycles of light and darkness. Instead, their sleep patterns appeared to track changes in the surrounding temperature.

"We had assumed that sunrise and sunset mattered because of light," Yetish said. "But warmth also comes from the sun."

Cooling Down for Sleep

Decades of research have shown that the body's core temperature drops as we prepare for sleep. For most people, body temperature continues to fall throughout the night, reaching a low point about 1°C (1.8°F) below its daytime peak.

Sebastian Herberger, a sleep specialist at the Berlin Center for Sleep Medicine, believes this helps explain why people tend to wake up more frequently and sleep for shorter periods when their bedrooms are too warm.

"Anyone who has tried sleeping in an overheated hotel room can easily understand this," Herberger said.

A 2007 study examined how changes in room temperature affect sleep. Participants slept under two different conditions.

In the first condition, the room temperature gradually dropped from 27°C (80.6°F) to 24°C (75.2°F) between 6 p.m. and 10 p.m. It then gradually rose back to 27°C between 3 a.m. and 7 a.m. This pattern was designed to mimic the natural cooling that occurs at night, followed by warming toward morning.

In the second condition, the room temperature remained constant at 27°C throughout the night.

The results showed that participants who slept in the room with changing temperatures had higher nighttime melatonin levels than those who slept in a room with a constant temperature. Melatonin is a hormone that signals to the body that it is time to rest and prepare for sleep. It is produced naturally, with levels rising as bedtime approaches and peaking during the night.

Participants who slept under changing temperature conditions also reported feeling more refreshed when they woke up.

Based on these findings, the researchers suggested that humans may have evolved to sleep more deeply in response to the natural temperature changes that occur around sunset and sunrise.

However, scientists have yet to determine exactly how cool the air needs to be for optimal sleep. They are also still investigating how strongly temperature influences the body's internal clock and the timing of sleep.

Resetting the Body's Internal Clock

The connection between temperature and sleep raises an important question: How much does temperature influence the body's internal clock?

The circadian rhythm regulates the physical, mental and behavioral changes that follow an approximately 24-hour cycle. It influences nearly every aspect of human activity, including when we sleep and eat. For a long time, scientists believed that the circadian rhythm was determined primarily by exposure to light.

Russell Foster, a professor of circadian neuroscience at the University of Oxford and author of Life Time, says the main reason body temperature drops before sleep is the circadian rhythm itself.

However, Foster cautions against viewing body temperature as merely a passive result of the internal clock. Temperature may also act as a stimulus that helps regulate that clock.

"Exactly what role temperature plays as a signal to the sleep system remains unclear," he added.

Animal studies also support the idea that temperature can help regulate the body's internal clock.

Research has shown that the circadian rhythms of a wide range of species, from fruit flies to camels, can adapt to regular fluctuations in temperature alone. Scientists have also found that simulated temperature changes can influence the internal clocks of human cells.

However, findings from animal and cell studies cannot automatically be applied to humans. Determining whether temperature is a key regulator of the human body clock remains challenging.

For example, if high temperatures make it difficult to fall asleep, the problem may have nothing to do with the circadian rhythm. The body may simply be unable to relax because the physiological conditions needed for sleep are not in place.

Still, a handful of small studies suggest that temperature may play a more direct role.

In a 1974 study, researchers kept the room temperature about 6°C (10.8°F) lower while participants were asleep than while they were awake. As a result, the participants' sleep-wake cycles lengthened by approximately 23 minutes.

The researchers concluded that "the effect of temperature on the control of human circadian rhythms is very similar to that of light."

A 2007 study also found that lowering and then raising the surrounding temperature directly affected participants' core body temperatures.

The time at which body temperature reaches its lowest point is an important indicator of the current phase of the circadian rhythm. Participants exposed to changing room temperatures reached their nightly minimum core temperature an average of about one hour earlier than those who slept in rooms with constant temperatures.

This suggests that bedroom temperature may directly contribute to resetting the body's internal clock.

Of course, much more research is needed to confirm these findings.

Christian Cajochen, a chronobiologist at the University of Basel, also agrees that temperature may help regulate the circadian rhythm. However, only a very small number of experiments have examined this possibility.

"It's too early to consider temperature the 'new light' when it comes to regulating the body clock," Cajochen said.

How Temperature Affects Sleep

Understanding the role of temperature in sleep is difficult because sleep itself is a complex physiological process involving multiple systems.

Two main systems determine when we feel sleepy and how strongly we feel the urge to sleep.

The first, known as sleep homeostasis, increases our need for sleep the longer we remain awake.

The second, the circadian rhythm, acts as an internal clock, regulating when we tend to feel sleepy and when we are more likely to be alert.

But a third system also has a major influence on sleep: the wake-promoting system. Under certain circumstances, it can override the other systems and keep us awake or wake us from sleep. This is why we can be jolted awake by a fire alarm even at 3 a.m.

These systems are interconnected. Yetish believes that the sleep patterns observed among the Tsimane, Hadza and San people may result from the interaction of these systems, along with other biological and environmental factors.

Jerome Siegel, a neuroscientist and director of the Sleep Research Center at the University of California, Los Angeles (UCLA), led research involving some of these Indigenous communities.

Siegel has suggested that temperature may be more important than light in regulating sleep. He believes this perspective has significant implications for modern health.

For example, Siegel suspects that modern homes, where temperatures remain relatively constant throughout the day and night, may contribute to sleep problems.

This possibility is consistent with observations that complaints of insomnia are much more common in industrialized societies than in nonindustrial societies.

When researchers investigated the prevalence of insomnia among the Tsimane and San people, they found that these communities did not even have a word for insomnia.

"Modern humans live apart from the natural environments in which we evolved," Siegel said.

More research will be needed to identify the best ways to use temperature to improve sleep quality.

For now, however, it may be worth trying something as simple as opening a window at night to let in some cool air—just as Yetish did when he was a child.

Source: BBC News 코리아 (https://www.bbc.com/korean/articles/cm93z9p0w4zwo)