STANFORD (CBS SF) – A new study by researchers at Stanford University is raising doubt about the normal human body temperature being 98.6 degrees Fahrenheit.

The study from the Stanford School of Medicine notes the average human body temperature among Americans has decreased since the 1800s, when the 98.6 standard was set. Researchers analyzed more than 677,000 temperature measurements over the last two centuries, from medical records of veterans who fought in the Civil War, to adult patients who visited the Stanford Health Care within the past decade or so.

Researchers found that the body temperature of men born in the 2000s has is 1.06 F lower on average than men born in the early 1800s. Meanwhile, the body temperature of women has also dropped, by 0.58 F on average.

The Stanford team hypothesized that the decrease in average temperature may be explained by a reduction in humans’ metabolic rate and lower inflammation.

“We speculated that the differences observed in temperature between the 19th century and today are real and that the change over time provides important physiologic clues to alterations in human health and longevity since the Industrial Revolution,” the authors said.

Researchers pointed to host of factors, such as advances in medical treatments, better hygiene and improved standards of living over the last two centuries.

“Physiologically, we’re just different from what we were in the past,” Dr. Julie Parsonnet, one of the study’s authors, said in a university statement. “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.”

Dr. Carl Reinhold August Wunderlich of Germany established the 98.6 degree Farenheit standard in 1851, after studying 25,000 patients.

The study was published in the biomedical science journal eLife.