Anthropologist Gandhi Yetish first became interested in the idea while studying sleep in non-industrial societies.
Research on the Tsimane of Bolivia, the Hadza of Tanzania and the San of southern Africa found that their sleeping patterns did not closely align with sunrise and sunset. Instead, they appeared to track daily temperature changes. In one widely cited study, researchers found that members of all three groups typically fell asleep during the coolest period of the day and woke up as temperatures reached their lowest point before beginning to rise.
"We assume that the reason sunset and sunrise are important is because of the light," Yetish said. "Well, temperature also comes from the Sun."
Why cooler temperatures help us sleep
Scientists have long known that core body temperature drops before sleep and continues to decline through much of the night, often by around 1C.
This helps explain why sleeping in a room that is too warm often leads to more disrupted sleep, according to sleep physician Sebastian Herberger.
Evidence suggests that environmental temperature may actively support this process. In a 2007 study, participants slept either in a room with temperatures that gradually cooled and then warmed overnight, mimicking natural outdoor conditions, or in a room kept at a constant temperature.
Those exposed to changing temperatures produced higher levels of melatonin, the hormone that helps regulate sleep. Melatonin naturally rises before bedtime and typically peaks during the middle of the night.
Participants in the fluctuating-temperature group also reported feeling more refreshed when they woke up.
Could temperature help set our body clocks?
The more controversial question is whether temperature merely affects sleep quality or whether it also helps regulate our circadian rhythms, the biological cycles that govern sleep, alertness, eating patterns and other daily functions.
Circadian rhythms are traditionally thought to be driven primarily by light exposure. According to the US National Institutes of Health, these rhythms involve a range of mental, physical and behavioural changes that repeat roughly every 24 hours.
Oxford University circadian neuroscientist Russell Foster argues that temperature may provide an additional signal influencing these rhythms.
Animal research has found that temperature cycles can synchronise biological clocks in species ranging from fruit flies to camels. Scientists have even managed to alter the timing of human cells' internal clocks by exposing them to temperature fluctuations in laboratory experiments.
What human studies show
Evidence in humans remains limited but intriguing.
A 1974 study found that lowering bedroom temperatures by around 6C during sleep altered participants' sleep-wake cycles by approximately 23 minutes. Researchers concluded that temperature's influence on circadian timing may be comparable to that of light.
The 2007 Japanese study also found that participants exposed to changing temperatures reached the lowest point of their body-temperature cycle roughly an hour earlier than when room temperatures remained constant. Because that low point is considered an important marker of circadian timing, the finding suggests bedroom temperature may help influence the body's clock.
Even so, many researchers remain cautious.
Chronobiologist Christian Cajochen of the University of Basel believes temperature probably affects circadian rhythms but says too few studies have been conducted to elevate it to the same status as light.
"In my view, temperature is not the 'new light'," he said.
Modern life may be working against us
For neuroscientist Jerome Siegel of UCLA, the implications stretch beyond the bedroom.
He argues that modern humans spend most of their lives in temperature-controlled environments that remain largely unchanged between day and night. That contrasts sharply with the conditions under which humans evolved.
Research has found significantly lower rates of insomnia in non-industrial societies than in industrialised ones. In the same study of Indigenous populations, researchers reported that neither the Tsimane nor the San had a word for insomnia.
"We just don't live in the environment that we've evolved for," Siegel said.
What we know, and what we don't
While the evidence linking cooler temperatures to better sleep is increasingly strong, scientists are still debating how much temperature influences circadian rhythms and exactly what temperature range is optimal.
For now, however, the research suggests that naturally cooler sleeping conditions and gradual overnight temperature changes may help promote deeper, more restorative sleep.
Source: BBC
