The role of temperature has gained attention since a study published last year found that core body temperature, which tends to fluctuate by a few degrees over the course of the day, needs to drop to help initiate sleep.
Setting the thermostat to around 18C is good for sleep, studies have found. Research has also found that room temperatures as low as 16C are best when people pile on the blankets.
Temperature is a big point of debate for couples. Women tend to raise the thermostat while men want to lower it. While researchers haven’t focused on such differences, many companies have with products from mattress that promise zoned temperatures to apps that let you control heating.
“People tend to set their ambient house or bedroom temperature a little higher than is actually optimal for sleep,” says Matthew Walker, a professor of neuroscience and psychology at the University of California, Berkeley.
The body’s core temperature needs to drop by about two to three degrees to initiate sleep, Walker says. “If our core temperature is too high, the brain cannot easily make the switch from being awake to being asleep, or create the best quality sleep.”
Core body temperature is the temperature of our heat-producing core, which is the brain and abdominal cavity. As the ambient temperature drops, so too does our core temperature. It usually reaches the lowest level in the early morning hours, before awakening.
When treating insomnia patients, sleep experts will often ask about room temperature and advise patients who set their thermostat to about 21C to drop it, Walker says. For people who live in hot climates and don’t have air-conditioning he recommends minimal bed clothes, a light bed cover and open windows.
Flinders University clinical psychologist Michael Gradisar says that during sleep, people’s bodies naturally try to lose heat from the hands and feet. Put on socks if your feet are too cold, he suggests. And if you’re too hot, try sticking your hands and feet out from under the covers.
A study published in October in the journal Current Biology that examined sleep patterns of pre-industrial societies found temperature played a critical role. The 94-person study suggested “the daily cycle of temperature change, largely eliminated from modern sleep environments, may be a potent natural regulator of sleep” says researcher Jerry Siegel, a professor of psychiatry at University of California, Los Angeles. The study looked at three groups living in tropical, natural environments.
Siegel says none of the study participants went to sleep near sunset or woke up at sunrise. On average they fell asleep three hours and two minutes after sunset and woke up before sunrise. They slept about one more hour in the winter than in summer.
After gathering temperature information, Siegel realised the sleep period in the Hadza people of Tanzania occurred during the coldest part of the night. In follow-up studies involving groups in Namibia and Bolivia, he found the participants consistently woke up when the early morning temperature stopped falling.
“Temperature may have a much greater role in helping promote normal sleep than we previously thought,” says John Peever, a professor in the department of cell and systems biology at the University of Toronto. He says specific brain cells located in a region called the hypothalamus sense temperature changes to control sleep.
In a 2008 study in the journal Brain, a group of researchers in the Netherlands put 24 people in a thermosuit that allowed them to manipulate temperature by running water through the veins of the suit. They found that a 0.4C increase in skin temperature — which allows the body to release more heat — led to fewer wake-ups and more slow-wave, or deep, sleep.
“By dialling down the temperature of the body (core), the participants in the experiment fell asleep quicker, and they also obtain significantly deeper quality of sleep,” says Walker, who wasn’t involved in the study.
Other experiments that varied the ambient temperature — decreasing it early in the night and increasing it in the morning — have shown similar benefits for improving and maintaining sleep.
Taking a hot bath before bed has a similar effect.
The hot water brings the circulating blood to the surface of the body, which is one of the quickest ways to drop core body temperature.
“When you get out of the bath you cool down more quickly, which is what the body wants to do at bed time,” says James Horne, a neuroscience professor at Loughborough University in England. His research has found that young, healthy people have about 10 per cent more slow wave sleep when they take a warm bath before bedtime.
He says soaking in water that is about 39C for 30 minutes in the early evening will improve sleep. A shower won’t have the same effect.
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