The radical idea that temperature sets when we sleep

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Even today, anthropologist Gandhi Yetish remembers the bedtime ritual he had as a child during cold New Jersey winters. He would open his window, letting the frigid air rush in and cool his face. He'd quickly feel drowsier, though he'd wonder what his mother would say about the heating bill.

There's emerging evidence that Yetish was onto something. From how deeply we sleep to how refreshed we feel afterwards, temperature can have a profound impact on our nightly rest. Some scientists are even going one step further, arguing that temperature may be as important as light for shaping when we sleep.

Here's what we do and don't know.

For years, Yetish thought his preference for cool sleeping quarters was a personal quirk. Then, while researching the sleep of people in non-industrial societies, he spent time with the Tsimane, an indigenous group in Bolivia's Amazon.

Yetish was keenly aware of the long-held scientific consensus that the most important environmental cue for sleep was light exposure. But he found that the Tsimane typically went to sleep hours after sunset and rose before sunrise, despite lacking electricity for artificial lighting and spending most of their days outdoors.

Next, Yetish and a team of colleagues from universities across the US investigated the sleep of two other indigenous groups. These included the Hadza of northern Tanzania and the San in Southern Africa. The researchers found that their sleep timings also didn't correspond with light and dark cycles. Instead, it seemed to be tracking the rise and fall of the ambient temperature.

"We assume that the reason the sunset and sunrise are important is because of the light," says Yetish. "Well, temperature also comes from the Sun."

Decades of evidence shows that our core body temperature begins to drop as we wind down for sleep. For most of us, it continues to decline for much of the night, usually falling by around 1C from its daytime high to nighttime low.

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This helps explain why people often experience shortened, more disturbed sleep when their room isn't cool enough, says Sebastian Herberger, a sleep physician and scientist at the Interdisciplinary Center of Sleep Medicine in Berlin. "Everyone who's ever slept in a hotel room that's too hot would potentially know these kinds of effects," he says.

For a 2007 study, researchers changed the temperature of the room participants slept in to investigate its effects. The group experienced two different sleep set-ups. In one experiment, they were provided with a room where the temperature lowered from 27C (81F) to 24C (75F) from 18:00 to 22:00, slowly rose to 27C between 03:00 and 07:00, and then remained at 27C during the day. This was to mimic the natural rise and fall of the ambient temperature through the night. In the other experiment, they slept in a room at a constant 27C.

When they slept in fluctuating temperatures, the participants had higher levels of a marker of melatonin overnight, compared to those who slept at a steady 27C. This hormone is produced by the body naturally, increasing in the lead-up to sleep time and usually reaching its peak in the middle of the night. It tells your body to wind down and prepare for sleep.

The same group also felt more refreshed when they awoke. The researchers speculated that we evolved to have deeper sleep under the influence of the natural temperature fluctuations at dusk and dawn.

However, the ideal temperature drop required to drift off to sleep is not yet clear – and how much temperature might impact our body clocks and sleep timings is still being debated.

One question raised by the potential link between temperature and sleep is whether, and how much, it can help to set our internal body clocks. These clocks coordinate our "circadian rhythms", cycles of mental, physical and behavioural changes that occur in any 24-hour period. This includes everything from when we sleep to when we eat. Circadian rhythms are traditionally thought to be dictated mainly by our exposure to light.

The circadian rhythm is the primary driver behind the fall in our body temperature in the lead-up to sleep, says Russell Foster, professor of circadian neuroscience at the University of Oxford and author of the book Life Time.

But Foster explains that temperature may not just be influenced by our body clocks – it may be an extra input that helps shape them, too. "What it's actually doing in terms of as a signal to the sleep systems, we don't know," he says.

Animal experiments have provided some evidence for the idea that temperature may itself be helping to set our body clocks.

Studies have found that you can train the circadian rhythms of a variety of species, from fruit flies to camels, through fluctuating temperature cycles alone. Scientists have also been able to train the internal clocks of human cells by simulating fluctuating body temperatures.

We just don't live in the environment that we've evolved for – Jerome SiegelAnimal and cell studies don't necessarily apply to real-life humans, and discerning whether temperature is really a key clock-setter for our species is remarkably difficult. For example, if someone has more trouble sleeping in warmer temperatures, that might have nothing to do with their circadian rhythm. Instead, it may be down to what their body physiologically needs to relax.

But the findings of some small studies hint that there could be more to it. In one 1974 study, participants adjusted their room temperature so that it was about 6C (11F) cooler while they were sleeping compared to when they were awake. That change lengthened their sleep-wake cycles by around 23 minutes – meaning that, when it comes to controlling human circadian rhythms, "the effect of temperature is very similar to that of light", the study noted.

Meanwhile, a 2007 study found that lowering then increasing the ambient temperature affected the participants' internal body temperature too. The timing of when your body temperature hits its lowest part is an indicator of where you are in your circadian rhythm. The participants exposed to fluctuating temperatures reached that low point about an hour earlier in the night, on average, than when the room temperature stayed constant. This suggests that the temperature of your bedroom can help to set your circadian clocks.

Temperature may help to modulate our circadian rhythms, says Christian Cajochen, a chronobiologist at the University of Basel. But very few experiments have explored the question. As a result, for now, "in my view, temperature is not the 'new light'" in terms of setting our body clocks, he says.

When it comes to temperature's role on sleep, what makes matters more complicated is that sleep is a multi-dimensional physiological process.

Two core systems determine when we get sleepy, and how much. The first, the sleep homeostat, means that the longer we are awake, the sleepier we get. The second, the circadian rhythm, is made up of a series of body clocks that time when we feel drowsy and when we are alert.

But a third system has an impact, too. This is the "arousal-promoting system", which, in the right circumstances, can override the other systems to keep you awake. This is the reason why a fire alarm is likely to get you out of bed, even if it is 03:00.

While all of these systems are linked together, Yetish acknowledges that the sleep patterns he studied in the Tsimane, San and Hadza could have been down to one of these other processes.

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Jerome Siegel, neuroscientist and director of the Center for Sleep Research at the University of California, Los Angeles, led the research on these indigenous groups. "I think temperature is more important than light," he says. In his view, this has wide implications for modern health.

For instance he argues that our tendency to live in homes where the temperature remains the same day and night is likely to contribute to the sleep issues people have in the modern world. This includes the far higher rates of insomnia reported in industrial societies compared to non-industrial ones.

When the researchers investigated the prevalence of insomnia among the Tsimane and San, they found that neither group had a word for the condition.

"We just don't live in the environment that we've evolved for," says Siegel.

More research is needed to figure out the best way to use temperature to hack your sleep. But for now, opening a window at night like Yetish used to might be worth a try.

*Amanda Ruggeri is a science, psychology and history journalist. She writes the six-part How to Sleep Better newsletter course, and her forthcoming book, The Sleep Myth, will be published in summer 2027.

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