A long-running study reanalyzed by researchers at SRI International finds women’s resting body temperature gradually increases from ages 18 to 42. The shift is small but measurable, tracks across both halves of the menstrual cycle, and may eventually help with
Somewhere between a thermometer’s click and the quiet routine of waking up each morning. a clue about aging has been sitting in plain sight. Now. researchers have reexamined decades-old temperature logs from hundreds of women and found a pattern that doesn’t match what many people assume about the body staying constant through adulthood.
Resting body temperature rises a little bit each year in women from the age of 18 to 42. The reason is still being figured out. The discovery matters because it suggests temperature-sensing wearables could one day be used not only to track reproductive health. but also to follow broader changes that come with getting older—and potentially flag when something is going off track.
Marie Gombert-Labedens. at SRI International. a research institute headquartered in California. says temperature signals may carry more health information than people have realized. “We think there is a lot of information about health in the temperature signal,” she says. “We hope this facilitates investigations to identify novel markers of conditions – it may be an untapped resource of information about our health.”.
The work builds from data collected in a 1990s study. More than 750 women aged 18 to 42 were asked to measure their oral or rectal temperature with a thermometer every day when they first woke up. In that dataset. the team saw a consistent menstrual-cycle effect: body temperature was lower during the first half of participants’ menstrual cycles and higher during the second half. after ovulation occurred. Many fertility-tracking apps already rely on this temperature jump to predict a user’s fertile window.
Gombert-Labedens and her colleagues went further than the cycle-level pattern. They re-examined the same temperature records in more detail to investigate the effect of age across different stages of the menstrual cycle. What emerged was a gradual shift over time. Each year from age 18 to 42, participants became a fraction warmer, on average.
That translated into a measurable difference by midlife. The researchers report that women aged 35 and older tracked about 0.05°C hotter than younger subjects across both halves of the menstrual cycle.
The finding is consistent with earlier work from the same team. In that previous research, finger skin temperature was measured continuously by a smart ring, and women aged 42 to 55 were warmer on average than those aged 18 to 35.
Still, the new results don’t answer the question most people will immediately ask: why does it happen?. Gombert-Labedens says further research is required to explain the temperature increase. but she suspects hormonal changes are involved. especially toward the end of the reproductive years. When perimenopause begins. temperature can increase suddenly and cause hot flushes and night sweats—but it remains unclear whether the gradual temperature rise seen from 18 to 42 is driven by the same mechanisms.
A key detail in the study design makes the mystery even more focused. 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). Because of those exclusions. researchers don’t yet know how hormonal contraception or those conditions might affect body temperature over time.
There’s also a broader comparison point from other research. After menopause, body temperature in women tends to drop back down to a lower level and become similar to that of men.
That rhythm—warming into midlife and then shifting again after menopause—may be part of why some women describe feeling less cold as they get older. Gombert-Labedens says the higher temperature around midlife may explain that anecdotal experience. “We speculate that the higher temperature in midlife women could influence their perception and reaction to environmental temperature,” she says.
The most practical promise in the research is tied to what wearables can do next. As smart rings and other temperature-sensing devices become more common. it may be possible to identify patterns or deviations in individuals’ temperature trends that signal the approach of menopause. The research also raises the possibility of estimating a person’s rate of biological ageing or detecting early signs of ovarian cancer or other conditions.
For now, the core takeaway is simpler and stranger than any wearable forecast: women appear to run warmer year by year from late adolescence into early midlife—even before perimenopause arrives—and scientists still don’t fully know why.
body temperature women’s health aging menstrual cycle ovulation perimenopause hot flushes night sweats smart rings wearables ovarian cancer