For over a century, we’ve believed that 36.6°C (98.6°F) is the average body temperature for a healthy person.

But a new study from Stanford University is challenging this long-standing belief.

The findings were surprising, as the study revealed that normal human body temperatures ranged from 36.2°C to 36.8°C (97.3°F to 98.2°F). This suggests that the traditionally accepted normal value of 36.6°C is actually too high.

Led by infectious disease expert Dr. Julie Parsonnet, the study reveals that the widely accepted average of 36.6°C is inaccurate. Dr. Parsonnet and her team analyzed 618,306 temperature measurements from patients at Stanford Health Care between 2008 and 2017.

They also recorded factors such as the time of day, the patient’s age, weight, height, body mass index, medications, and health conditions.

"Many people, including many doctors, still believe that 98.6°F is everyone's normal temperature. In reality, what's considered normal varies from person to person and situation to situation, and it’s rarely as high as 98.6°F," said Dr. Parsonnet in a statement.

The study, published in JAMA Internal Medicine, suggests a cooling trend, though the significant factors of age, time of day, and gender prevent an updated definition of 'average body temperature' that would apply to all Americans today.

The decline in average body temperature in the U.S. may be linked to metabolic changes or the energy used by the body. The researchers noted that a reduction in inflammation across the population could be contributing to this decrease. "Inflammation produces proteins and cytokines that elevate metabolism and raise body temperature," Dr. Parsonnet explained.

Additionally, the dramatic improvements in public health over the past two centuries—including advancements in medical care, better hygiene, wider food access, and improved living standards—may have played a significant role.

"Physiologically, we're different from what we were in the past. Our environment has evolved, from the temperature in our homes to our exposure to microorganisms and the food we consume.

These changes indicate that, although we tend to view humans as unchanging throughout evolution, we are actually undergoing physiological changes," said Parsonnet.

"There is a wealth of temperature data globally, offering a valuable opportunity to learn more," she added. Parsonnet also suggested that future studies could explore personalized definitions of fever and investigate whether having a consistently higher or lower normal body temperature impacts life expectancy.