Resting body temperature rises a little bit each year in women from the age of 18 to 42, for reasons that are still being figured out. The finding opens up the possibility of using temperature-sensing wearables to track ageing and detect perimenopause or potential health problems.

“We think there is a lot of information about health in the temperature signal,” says Marie Gombert-Labedens at SRI International, a research institute headquartered in California. “We hope this facilitates investigations to identify novel markers of conditions – it may be an untapped resource of information about our health.”

Gombert-Labedens and her colleagues analysed data from a 1990s study that asked more than 750 women aged 18 to 42 to measure their oral or rectal temperature with a thermometer every day when they first woke up.

The readings showed that, on average, body temperature was lower during the first half of the participants’ menstrual cycles and higher during the second half, after ovulation occurred. Many fertility-tracking apps use this temperature jump to predict a user’s fertile window.

Gombert-Labedens and her colleagues re-examined the data in more detail to investigate the effect of age on temperature across different stages of the menstrual cycle. They found that each year from age 18 to 42, the participants became a fraction warmer, on average. As a result, those aged 35 and older tracked about 0.05°C hotter than the younger subjects across both halves of the menstrual cycle.

This is consistent with the team’s previous research that found that finger skin temperature, measured continuously by a smart ring, was warmer on average in women aged 42 to 55 than in those aged 18 to 35.

Further research is required to explain why this temperature increase occurs, but it probably has something to do with hormonal changes, especially towards the end of the reproductive years, says Gombert-Labedens. When perimenopause begins, temperature can increase suddenly and cause hot flushes and night sweats, but it’s unclear whether this is linked to the same mechanisms, she says.

The study only included women who did not use hormonal contraception and did not have hormonal conditions such as PMOS (polyendocrine metabolic ovarian syndrome, previously known as PCOS), so we don’t yet know how these factors might affect body temperature over time.

Other research has found that after menopause, body temperature in women tends to drop back down to a lower level and become similar to that of men.

The higher temperature around midlife may explain why anecdotally, some women at this age say they don’t feel the cold as much as they used to, says Gombert-Labedens. “We speculate that the higher temperature in midlife women could influence their perception and reaction to environmental temperature,” she says.

As smart rings and other temperature-sensing wearables become more popular, it may be possible to identify patterns or deviations in individuals’ temperature trends that signal the approach of menopause, provide an estimate of their rate of biological ageing or detect early signs of ovarian cancer or other conidtions, says Gombert-Labedens.