Ask almost anyone what a normal body temperature is, and they'll say 98.6°F without blinking. It's printed in textbooks, taught in schools, and checked against every time a child feels warm. But where did that number come from, and is it actually right?
Here's the short answer. The 98.6 figure dates back to the 1800s, and newer studies suggest the average internal temperature is a bit lower, around 97.5°F. More importantly, there's no single “normal.” Your temperature shifts with the time of day, your age, how you measure it, and what you're doing.
Where 98.6 Came From
The number traces back to German physician Carl Wunderlich, who published a book in 1868 that put temperature charts into everyday medical use. He set the normal value at 37 degrees Celsius.
There's a fun twist. Before that, many books listed “blood heat” as 98°F. Later, an average from a large study was widely quoted as “37°C or 98.4°F,” until editors realized that 37°C actually converts to 98.6°F. Sources often added the word “about,” but rarely said how much wiggle room there really was.
Your Built-In Thermostat
Your body works hard to keep its temperature in a healthy range. The control center is a small part of the brain called the hypothalamus, which acts like a thermostat. It sets a target temperature and then adjusts.
If you get too warm, it triggers sweating and widens blood vessels in the skin to release heat. If you get too cold, it does the opposite, producing and holding on to heat. A fever happens when that thermostat's setting is raised, usually because of an infection.
Why Your Number Keeps Changing
Even in a healthy person, temperature rises and falls by about 0.9°F over the day. It's usually lowest in the morning, with the low point coming about two hours before you normally wake up, and highest in the late afternoon and evening.
Where you measure matters too. Readings under the tongue average about 98.2°F, while readings taken inside the body run a little higher, closer to 98.6°F. Ear and underarm readings give different numbers again. Hot coffee or a cold drink can throw off a mouth reading.
Hormones play a part, too. A woman's resting temperature rises noticeably after ovulation, and hormonal birth control can raise typical body temperature by about 1°F. Your temperature also shifts when you're hungry, sleepy, sick, or cold.
Age plays a role as well. As people get older, both their average temperature and its daily ups and downs tend to shrink. Older adults may not run as high a fever when sick, so even a slightly raised reading can matter more. If you're ever unsure about a reading, talk to your doctor.
Are We Getting Cooler?
Here's a surprising wrinkle. One study suggested that average body temperature has dropped since the 1850s, which would mean Wunderlich's patients really did run a little warmer. The researchers think the most likely reason is less inflammation across the population, thanks to fewer long-lasting infections and better hygiene.
So if your thermometer rarely reads exactly 98.6, there's no need to worry about the number itself. That old figure was a reasonable average for its time, not a rule your body has to follow. Like the rest of us, your temperature has a little personality of its own.
