---
title: "The thermostat and the dawn: does temperature, not light, rule our sleep?"
description: "From Amazonia to the sleep labs of Berlin, scientists are asking whether we set our body clocks by the cold"
author: "Jan Sokol"
published: 2026-10-08T09:01:27.003Z
modified: 2026-10-08T21:07:23Z
url: https://rews.cc/a/the-thermostat-and-the-dawn-does-temperature-not-light-rule--68b7ba
language: en
tags: ["sleep", "circadian", "temperature", "biology", "science"]
publisher: "Rews (https://rews.cc)"
---

# The thermostat and the dawn: does temperature, not light, rule our sleep?

*From Amazonia to the sleep labs of Berlin, scientists are asking whether we set our body clocks by the cold*

By Jan Sokol · October 8, 2026 · https://rews.cc/a/the-thermostat-and-the-dawn-does-temperature-not-light-rule--68b7ba

## In brief

- Gandhi Yetish found Bolivia’s Tsimane fell asleep hours after sunset despite having no electric light
- Core body temperature usually falls about 1C from daytime high to nighttime low as we sleep
- A 2007 study found naturally fluctuating room temperatures raised overnight melatonin and refreshed sleepers
- A 1974 study showed cooler sleeping quarters lengthened sleep-wake cycles by around 23 minutes
- UCLA’s Jerome Siegel argues temperature beats light; the Tsimane and San have no word for insomnia

As a boy in New Jersey, during winters that froze the window frames, the anthropologist Gandhi Yetish had a private ritual. He would open the window before bed and let the frigid air rush over his face. Drowsiness came quickly. Somewhere behind the drowsiness was a guilty thought about his mother and the heating bill. As [Amanda Ruggeri reports for the BBC](https://bbc.com/future/article/20261007-how-does-temperature-affect-your-sleep), the boy was onto something that science is only now catching up with: temperature, some researchers argue, may be as important as light in deciding when we sleep.

## Sleeping without clocks

For years the consensus was simple: light exposure is the supreme environmental cue for sleep. Then Yetish went to live with the Tsimane, an indigenous people of Bolivia’s Amazon. They had no electricity and spent their days outdoors, drenched in natural light — yet they typically fell asleep hours after sunset and rose before the sun. Light was not commanding them. With colleagues from universities across the US, Yetish next studied the Hadza of northern Tanzania and the San of Southern Africa. Their sleep timing also ignored the light-dark cycle. It tracked, instead, the rise and fall of ambient temperature. “We assume that the reason the sunset and sunrise are important is because of the light,” Yetish says. “Well, temperature also comes from the Sun.”

## The cool descent

Decades of evidence say the body prepares for sleep by cooling: core temperature begins to fall as we wind down and keeps falling much of the night, usually by around 1C from daytime high to nighttime low. That is why a stuffy room frays sleep, says Sebastian Herberger of Berlin’s Interdisciplinary Center of Sleep Medicine: “Everyone who’s ever slept in a hotel room that’s too hot would potentially know these kinds of effects.”

In a 2007 study, researchers gave sleepers two kinds of nights. In one, the room cooled from 27C (81F) to 24C (75F) between 18:00 and 22:00, warmed back to 27C between 03:00 and 07:00, and stayed there by day — an imitation of nature’s night-time curve. In the other, the room sat at a constant 27C. Sleepers in the fluctuating room had higher overnight levels of a melatonin marker — melatonin being the hormone the body produces as bedtime approaches, peaking in the middle of the night — and woke feeling more refreshed. The researchers speculated that we evolved to sleep deeply under the natural temperature swings of dusk and dawn. How big a drop is ideal remains unknown, and how far temperature steers our body clocks is still debated.

## Winding the clock

Our circadian rhythms — the cycles of sleeping, waking, eating, alertness that repeat every 24 hours — are traditionally thought to be dictated mainly by light. Russell Foster, professor of circadian neuroscience at the University of Oxford and author of *Life Time*, says the circadian rhythm is the primary driver of the temperature drop before sleep. But causality may run both ways; temperature may be an input that shapes the clock as well as an output of it. “What it’s actually doing in terms of as a signal to the sleep systems, we don’t know,” he says.

Animal experiments offer hints that temperature can set clocks, though animals and cells are not people, and the human evidence is small and old and tantalising. In a 1974 study, participants kept their rooms about 6C (11F) cooler while asleep than awake — and their sleep-wake cycles lengthened by around 23 minutes, leading the study to note that “the effect of temperature is very similar to that of light.” A 2007 study found that sleepers in fluctuating temperatures reached their body’s nightly low point about an hour earlier, on average, than at constant warmth — and since the timing of that low marks where you are in your circadian rhythm, the bedroom itself appeared to be moving the hands of the clock. Christian Cajochen, a chronobiologist at the University of Basel, is unswayed for now: too few experiments exist, “in my view, temperature is not the ‘new light’”

## The alarm that overrides everything

Sleep, complicating everything, is multi-dimensional. Two core systems govern it: the homeostat, which makes you sleepier the longer you are awake, and the circadian rhythm, the clockwork itself. But a third, the “arousal-promoting system”, can overrule both — the fire alarm at 03:00 will have you upright in seconds. Yetish concedes that in the Tsimane, San and Hadza camps, these other processes might explain the patterns he saw.

> We just don’t live in the environment that we’ve evolved for.

Jerome Siegel, the UCLA neuroscientist who led the indigenous-sleep research, has no such caution. “I think temperature is more important than light.” In his view, sealed, climate-constant homes are implicated in modern sleep misery — including the far higher insomnia rates reported in industrial societies. When his team asked after insomnia among the Tsimane and the San, they found neither group even had a word for it. A condition we complain about endlessly, and whole peoples have never needed to name.

More research must settle how best to weaponise the thermostat. Until then, the prescription is the one a boy in New Jersey wrote for himself, five decades and half a world ago: open the window, let the cold in, and let the body do what it learned to do at dusk.