Scientists say cooling down at night could be as important as darkness for good sleep

Most people know that darkness signals the brain to wind down for sleep. But a growing body of research suggests another environmental cue may be just as powerful -- and far less understood: temperature.

Scientists studying sleep in non-industrial communities have found that people tend to fall asleep not when the sun goes down but when the surrounding air cools, reports BBC.

Researchers who spent time with the Tsimane people of Bolivia’s Amazon, the Hadza of northern Tanzania and the San of southern Africa found that despite having no electric lighting and spending most of their days outdoors, none of these groups timed their sleep to align neatly with light and dark cycles.

Instead, their sleep appeared to track the natural rise and fall of ambient temperature through the day.

“We assume that the reason sunset and sunrise are important is because of the light,” said anthropologist Gandhi Yetish, who was part of the research team. “Well, temperature also comes from the Sun.”

Why the Body Needs to Cool Down

The link between temperature and sleep is partly physiological. Core body temperature naturally begins to drop as we approach sleep, falling by around 1C from its daytime peak to its nighttime low.

That cooling process appears to be important for settling into deep, restorative rest.

A study that manipulated bedroom temperatures found that participants who slept in rooms where the temperature fell and rose naturally through the night had higher levels of melatonin -- the hormone that signals the body to prepare for sleep -- compared to those who slept in a room held at a constant temperature, according to BBC.

They also reported feeling more refreshed upon waking.

Sleep physician Sebastian Herberger, based in Berlin, said the effects of sleeping in an overly warm room were something many people had experienced firsthand. A hotel room that never cools down, he noted, was a common culprit behind a restless night.

Reshaping the Body Clock

The more ambitious scientific question is whether temperature does not just affect sleep quality but actively helps to set the internal body clocks that govern our circadian rhythms -- the roughly 24-hour cycles that regulate everything from sleep to appetite to mood.

Circadian rhythms have long been thought to be driven primarily by light exposure. But some researchers now believe temperature may be a significant input as well.

Animal studies have shown that circadian rhythms in species ranging from fruit flies to camels can be trained through temperature fluctuations alone. Scientists have also managed to shift the internal clocks of human cells in the lab using simulated body temperature changes.

In human studies, the evidence remains limited but suggestive.

One experiment found that participants who slept in a cooler room reached the lowest point of their nightly body temperature -- a key marker of where they are in their circadian cycle -- roughly an hour earlier than those who slept in a room held at a constant temperature.

Jerome Siegel, a neuroscientist at the University of California, Los Angeles, who led the indigenous sleep studies, believes temperature may ultimately prove more important than light.

He argues that the modern habit of living in homes where temperature stays constant around the clock is likely contributing to the high rates of insomnia seen in industrialised societies. Notably, neither the Tsimane nor the San had a word for insomnia.

Not all scientists are ready to go that far. Christian Cajochen, a chronobiologist at the University of Basel, said temperature likely played a role in modulating circadian rhythms but cautioned that far more research was needed before it could be considered on a par with light.

For now, the simplest takeaway may be the most practical: letting the bedroom cool down at night is unlikely to hurt and may well help.