Battle of the thermostat: The science behind your central heating arguments

Being too hot or too cold can be bad for your health. So, what is the optimum temperature for your home?

As someone who feels the cold – but is reluctant to constantly turn up the thermostat – a hot water bottle and a thick fleece are two of my closest friends. And as relentless winter storms sweep the east coast of the US and Canada – and other countries across the globe are trawling through what feels like a very long winter – you might also be wondering: what is the best indoor temperature for your health and comfort? The answer, it turns out, is not straightforward.

In the 1860s, German physician Carl Wunderlich took the temperatures of around 25,000 people and found that the average was 37C (98.6F). This figure has been widely accepted ever since. However, new data suggests that this isn't strictly true. Body temperature can vary between individuals, with modern research suggesting that gender, metabolism, hormones, activity, age and menstruation all have an influence. So, with such variations to consider, is there an optimum temperature at which to keep your home?

For countries with temperate or colder climates, guidance from the World Health Organization states that a room temperature of 18C (64.4F) is safe enough to protect us from the harmful effects of cold. Being in a cold, damp or draughty environment has negative physical and mental health consequences, from worsening asthma and increased risk of heart disease, to anxiety and depression. 

"We recognise healthy homes are one of the key building blocks for good health. So that means homes that are accessible, safe, comfortable and warm help to protect our health, and support our communities to be healthy and resilient," says Rebecca Wilson, senior public health specialist at Public Health Wales in the UK, an organisation which advocates for improving links between homes and health.

The results of Public Health Wales's 2022 and 2023 winter surveys found that people were increasingly experiencing stress or anxiety in the three months leading up to winter due to rising heating costs (a jump from 57.2% to 66.5%). When the cold set in, more people set their thermostats to lower temperatures at home, and chose to heat specific rooms rather than the whole house – and, instead, turned to hot water bottles or blankets to stay warm.

"We know that all people who live in cold homes or face frugal poverty are affected. But the research does show that there's certain population groups such as older people, children, and people with disabilities and health conditions, who might be more impacted," says Wilson. (Read more about how cold homes can affect health).

Equally, overheating is also a health risk for those living in warmer climates, with an increased likelihood of heat exhaustion and heatstroke as the body desperately tries to cool itself, in turn putting stress on the heart and kidneys. Overheating can also exacerbate existing health conditions and negatively impact mental health, with links found to an increased suicide risk. 

It's a bleak picture. Research has shown a global link between both cold and heat exposure and mortality over the 21st Century, with experts calling for more public awareness of non-optimal temperatures as a health risk. This is becoming an increasing problem due to climate change and a longer duration and intensity of extreme weather events.

Exposure factors like occupation and socioeconomic status can play a role – outdoor manual workers often work through extreme temperatures and people living in poorer urban or rural areas may be subject to low quality, draughty housing, or in hot weather, have less access to cooling like air conditioning.

Babies, young children and elderly people are also particularly vulnerable as they are less able to thermoregulate – meaning they are less able to maintain a core body temperature by balancing heat generation with heat loss. 

For example, according to The Lullaby Trust in the UK, the recommended room temperature for babies is slightly lower than average, at 16-20C (60-68F). If babies overheat, there is a risk of sudden infant death syndrome, so the charity advises using a thermometer to test the temperature of the room but also check your baby by feeling the back of their neck and chest to see if they are hot or sweaty. 

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The average body temperature of a newborn or a young child is slightly higher than an older child or adult, usually between 36.5 and 37.5C (97.7F and 99.5F). This is because their metabolism is more active, and their body surface area is larger in relation to their body weight. As their bodies grapple with thermoregulation, it takes longer for them to sweat, so their bodies hold more heat. 

Elderly people, on the other hand, may need their homes a little warmer as they tend to have a lower resting body temperature compared to young adults. Metabolic rate decreases with age, meaning it's harder for older people's bodies to produce heat. They are also more likely to be taking medication, or be suffering from a chronic illness – both of which can affect the body's ability to thermoregulate. Diabetes, for example, can prevent the blood flowing normally to provide warmth. Even slightly cooler homes at 18C (65F) can lead to hypothermia.

One study observed that the physical performance of older women between the ages of 70 and 95 in a moderately cold environment at 15C (59F) and warmer at 25C (77F). Results showed that their physical ability significantly worsened within 45 minutes at the colder temperature – highlighting the potential safety risks for older adults living independently, such as balance problems and heightened risk of falling. 

Men and women also have biological and hormonal differences, so our sensitivity to warmer and colder temperatures will differ too. Women tend to have more fat between their skin and muscles, so while the organs are kept warm, it prevents heat from reaching the skin and extremities (hands, feet and the tip of the nose). One study found that women's hands were almost 3C (5.4F) cooler than men's when exposed to cold – perhaps explaining where the phrase "cold hands, warm heart" comes from. 

Menstruation also has a role to play, as core body temperature fluctuates throughout the cycle, hitting its highest point in the luteal phase after ovulation. And pregnancy and hormonal contraceptives containing progesterone can also increase core temperature. However, as women are typically smaller in build and have a lower resting metabolic rate than men, which reduces heat production when exposed to the cold, they can also feel the cold more, despite having a higher core body temperature.

The extent of the variation of thermal comfort between men and women is disputed. While some research indicates that it is marginal, one study from Finland suggests that it might be more significant. Take the office environment, for example. While it's unlikely that there's a temperature which pleases everyone due to our individual thermoregulation, overall the study found that women were often dissatisfied with the room temperature, feeling both uncomfortably colder and hotter than men. Another study of office temperatures in Japan showed a 3.1C (5.6F) difference in neutral temperature between female and male workers.

Having a safe temperature in the workplace is crucial both for health and productivity. Heat stress can reduce physical work capacity and motor-cognitive performances, and workers – both outdoor and those in poorly insulated office buildings – exposed to cold environments can be at risk of injury, cold stress and illness. In the UK, the law requires employers to provide a reasonable indoor temperature in the workplace. Regulations suggest this should be 16C (60F) or 13C (55F) if the work involves "rigorous physical effort". In the US, however, while the Occupational Safety and Health Administration recommends a work temperature in the range of 20-24C (68-76F), it does not require employers to provide heat or air conditioning for workspaces. 

In the home, meanwhile, reaching an optimal core body temperature may not be a "golden ticket" to a perfect sleep, says Bill Wisden, a sleep expert and director of the UK Dementia Research Institute at Imperial College London, "but it would help".

Sleep, he says, is "temporally related to the circadian rhythm of core body temperature and skin temperature". In other words, a couple of hours before we go to sleep, our core body temperature begins to decrease when melatonin, the hormone that regulates the sleep cycle, is produced. Body temperature continues to drop throughout the night reaching a low point in the early morning before gradually warming up as the morning progresses.

One thing that can help you fall asleep is the "hot bath effect". Wisden says that having a hot bath before bedtime can help induce sleep. "We've done physiological experiments with mice and found that if you warm the skin, that sends signals to the hypothalamus in the base of the brain – and that triggers an induction of sleep and a cooling of the body." 

So, getting under a thick duvet at night can help the body prepare the body for longer episodes of sleep or deeper sleep. Snuggling up like this – to warm the skin – is not unique to humans. Wisden adds: "Pets, cats and dogs curl up – and some animals build a nest… that provides a kind of thermal insulation." 

But once you're fast asleep, a cool room temperature is ideal: between 16-18C (60-65F). While older adults might sleep more comfortably in a room that's a couple of degrees warmer, being in a room that's too warm can cause poor sleep quality. This can lead to waking up throughout the night, as well as a lack of non-REM sleep – the most restful stage of sleep. "Being too hot during sleep is bad. It's exactly like a noise, or a sensation on your skin, or a pain. Being hot while you're asleep stimulates sensory perception and wakes you up," Wisden says. 

Ultimately, while we can be guided by research, it seems it may be impossible to find an optimal temperature that will please everyone. From age to gender, hormones to metabolism, there are many factors at play that might influence whether you prefer a hotter or colder room.

So, the battle of the thermostat rages on.

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