In 1851, a German doctor named Carl Reinhold August Wunderlich took the temperatures of some 25,000 patients in the city of Leipzig and concluded that the average human body temperature sits at 37 degrees Celsius, or 98.6 degrees Fahrenheit. Though variations are known to exist from person to person, Wunderlich’s standard remains a benchmark for medical professionals today. But a new study published in eLife suggests that in the United States, at least, average temperatures are going down—a trend that can be observed in medical records spanning more than 150 years.

Previous research indicated that Wunderlich’s average may have run a little high. In a 1992 study of 148 patients, for instance, scientists at the University of Maryland measured an average temperature of 36.8 degrees Celsius, or 98.2 degrees Fahrenheit. More recently, a 2017 study of around 35,000 British patients found that the mean oral temperature clocked in at 36.6 degrees Celsius, or 97.9 degrees Fahrenheit. Some experts concluded that Wunderlich’s measurements had simply been inaccurate. But according to the new paper, authored by researchers at the Stanford University School of Medicine, human bodies are actually cooling.

The team looked at three large temperature datasets from three distinct periods. The first was compiled from medical records, military records and pension records of Union Army veterans; the data was obtained between 1862 and 1930. The researchers also consulted measurements from the U.S. National Health and Nutrition Examination Survey I, spanning from 1971 to 1975, and the Stanford Translational Research Integrated Database Environment, which consists of data from adult patients who visited Stanford Health Care between 2007 and 2017.

In total, the researchers studied 677,423 temperature measurements, collected over the course of 157 years and covering 197 birth years. They found that men born in the early 19th century display temperatures 0.59 degrees Celsius higher than men today, representing a decrease of 0.03 degrees Celsius per birth decade. Women’s temperatures have gone down 0.32 degrees Celsius since the 1890s, representing a 0.029 degree Celsius decline per birth decade—a rate similar to the one observed among male patients.

Was this a true cooling trend, or could the discrepancies simply be chalked up to improvements in thermometer technology? To find out, the scientists looked for patterns within each dataset, assuming that similar thermometers were used to take temperatures during a given historical period. Sure enough, they observed that measurements decreased at a similar rate. When it came to veterans of the Civil War, for instance, temperatures were higher among people born earlier, decreasing by 0.02 degrees Celsius with each birth decade.

“In previous studies people who found lower temperatures [in more recent times] thought the temperatures taken in the 19th century were just wrong,” study co-author Julie Parsonnet, a professor of medicine at Stanford’s School of Medicine, tells Alice Park of Time magazine. “I don’t think they were wrong; I think the temperature has gone down.”

According to the researchers, there are several reasons why our bodies might be cooling down. One is improvements in temperature regulation. “We have air conditioning and heating, so we live more comfortable lives at a consistent 68°F to 72°F in our homes,” Parsonnet explains. “[I]t’s not a struggle to keep the body warm.”

A more significant factor might be the reduction of inflammation-causing conditions like tuberculosis, malaria and dental diseases, thanks to improvements in medical treatments, hygiene standards and food availability. “In the mid-19th century,” the study authors note as an example, “2–3 percent of the population would have been living with active tuberculosis.” And the prevalence of such diseases may have had a population-level impact on average temperatures.

“Inflammation produces all sorts of proteins and cytokines that rev up your metabolism and raise your temperature,” Parsonnet says.

Not all experts are convinced by the study’s conclusions. Philip Mackowiak, who co-authored the 1992 study on average body temperatures, tells Nature’s Ewen Callaway that there are “so many variables that are unaccounted for”—like whether temperatures among the Civil War cohort were taken orally or in the armpit, which can produce different readings for the same person.

“There’s no biological explanation that I find convincing,” Makowiak says. “We’re talking about 200 years, which in the evolution of life is just a blink of the eye.”

But Parsonnet doesn’t think it’s a stretch to say that human physiology would change in response to a rapidly shifting environment.

“The environment that we’re living in has changed, including the temperature in our homes, our contact with microorganisms and the food that we have access to,” she says. “All these things mean that although we think of human beings as if we’re monomorphic and have been the same for all of human evolution, we’re not the same. We’re actually changing physiologically.”