In 1868, German physician Carl Reinhold August Wunderlich started to popularize what’s become the most recognizable number in all of medicine: 98.6°F or 37°C, which is thought to be the normal average human body temperature. Though his methods later came under scrutiny—Wunderlich stuck an enormous thermometer under the armpits of patients for 20 minutes, a less-than-accurate technique—this baseline has helped physicians identify fevers as well as abnormally low body temperatures, along with corresponding illnesses or diseases.

More than 150 years later, 98.6° may no longer be the standard. Humans seem to be getting cooler. Researchers at Stanford University School of Medicine, in a paper published in the journal eLife, compared three large datasets from different time periods: American Civil War records, a national health survey from the 1970s, and a Stanford database from 2007-2017. By comparing recorded body temperatures, the researchers founds that men are now averaging a temperature .58°C less than what's long been considered normal, while women are .32°C lower. On average, each has decreased roughly .03°C every decade since the 1860s.

What drove us to chill out? Scientists have a few theories. A number of advances in human comfort have been ushered in since the 1800s, including better hygiene and readily available food, which may have slowed our metabolic rate (temperature is an indication of that rate). Chronic inflammation, which also raises body temperature, has decreased with the advent of vaccines, antibiotics, and better healthcare. The researchers propose that, on average, our bodies are healthier and slightly less warm.

After all, the average life expectancy in Wunderlich’s era was just 38 years.

[h/t The Independent]