For centuries, 98.6 degrees Fahrenheit was said to be the average normal body temperature. It's not.

More recently, researchers have known normal body temperature is actually lower than that and can vary by gender, size, age, time of day and other factors. But now there's also new evidence that shows we've been cooling off since the 19th century when 98.6 was established as "normal."

Research published recently in eLife shows that body temperature has not only dropped since German physician Carl Reinhold August Wunderlich's study in 1851 established the average body temperature as 98.6 degrees, but it has also dropped since the 1970s. The findings indicate that Americans' average, normal body temperature has dropped about 0.05 degrees Fahrenheit per decade, based on their birth year.

"People are stuck on the 98.6 number, but that number has always been wrong," said Dr. Julie Parsonnet, one of the authors of the study and professor of medicine at Stanford University. "There's never been a real number because people vary."

In the study, Parsonnet and other researchers looked at the temperatures of three groups: data from a study that recorded Civil War veterans' temperatures from the mid-1800s through 1930, more recent data from the 1970s recorded by the Centers for Disease Control, and temperatures of patients visiting Stanford health clinics from 2007 to 2017.

The data showed that the body temperature of men born in the 2000s is about 1.06 degrees Fahrenheit lower than men born in the early 1800s. And the body temperature of women born in the 2000s is on average 0.58 degrees Fahrenheit lower than women born in the 1890s.

It's unclear what exactly is causing the continued decline, Parsonnet said.

Some factors could be that people have grown taller and heavier, and that their metabolic rates have slowed. Modern medicine's elimination of certain diseases, like syphilis, tuberculosis and periodontal diseases also could affect changes in body temperature, she said. When the original 98.6 degree normal was established, a significant portion of the population would've been fighting these diseases, Parsonnet said, which cause inflammation and higher temperatures.