The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) number most of us learned growing up. That familiar figure dates back to 1868, and mounting evidence shows that human bodies have been running cooler ever since. Your own normal temperature can vary by more than a full degree depending on the time of day, your age, and where on your body you measure it.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published his landmark work in 1868 after collecting over one million armpit temperature readings from roughly 25,000 patients. He identified 37.0°C (98.6°F) as the mean temperature of healthy adults, and that number stuck. It became the textbook answer taught in schools and used in clinics for more than 150 years.
The problem is that Wunderlich’s measurements were taken under the arm, which reads lower than oral temperature. His thermometers were also less precise than modern ones. And perhaps most importantly, the people he studied lived in a very different world, one with higher rates of chronic infection and inflammation, no antibiotics, and different living conditions. All of that influenced their baseline temperature.
Why Humans Are Running Cooler
A large study published in eLife analyzed temperature records spanning nearly 200 years of American history and found that average body temperature has dropped steadily over time: about 0.03°C per decade of birth. For men born between the early 1800s and the late 1990s, that adds up to a total decline of roughly 0.59°C (about 1.06°F). For women measured between 1890 and 1997, the decline was about 0.32°C (0.58°F).
The reasons likely involve broad improvements in public health. Lower rates of chronic infection mean less ongoing inflammation, which directly affects metabolic heat production. Better nutrition, climate-controlled environments, and widespread use of anti-inflammatory medications may all play a role. Metabolic rate, which generates body heat, tends to be higher in people with more muscle mass and higher body weight, but population-level changes in activity and inflammation appear to matter more than individual body composition for explaining this long-term cooling trend.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm. You’re coolest in the early morning hours, often dipping below 97.5°F, and warmest around 4 p.m., when readings can climb above 99°F in perfectly healthy people. This swing of roughly 1°F or more is driven by your circadian clock, the same internal timer that regulates sleep, hormone release, and alertness.
This matters when you’re checking for a fever. A reading of 99.0°F at 6 a.m. is more significant than the same number at 4 p.m. If you’re trying to establish your own baseline, take your temperature at the same time of day for several days in a row.
Where You Measure Makes a Difference
Not all thermometer readings are created equal. Oral temperature (under the tongue) is the most common reference point, but different body sites give different numbers:
- Rectal: Reads 0.5 to 1°F higher than oral
- Ear (tympanic): Reads 0.5 to 1°F higher than oral
- Armpit (axillary): Reads 0.5 to 1°F lower than oral
So an armpit reading of 97.6°F and a rectal reading of 99.6°F could both reflect the same core temperature. This is especially important for parents checking a child’s temperature, since the method you use determines what counts as a fever.
What Counts as a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. That threshold is based on oral measurement. Given that normal body temperature for most adults now sits closer to 97.9°F, a reading of 100.4°F represents a rise of about 2.5°F above baseline, which is a meaningful jump your immune system is driving on purpose to fight infection.
If you can’t take an accurate reading, the CDC also recognizes feeling warm to the touch, a flushed face, chills, or a self-reported history of feeling feverish as reasonable indicators. Keep in mind that medications like ibuprofen or acetaminophen can temporarily mask a fever by lowering your measured temperature without resolving the underlying cause.
Factors That Shift Your Baseline
Stanford Medicine researchers found that time of day has the single biggest influence on any given temperature reading. But several other factors shape your personal normal. Age is one of the most significant: metabolic rate tends to decline as you get older, and with it, baseline temperature. Older adults often run noticeably cooler than younger ones, which can make fevers harder to detect in that population.
Sex plays a role as well. Women’s body temperatures fluctuate with the menstrual cycle, rising about 0.5°F after ovulation due to hormonal shifts. Physical activity temporarily raises temperature, sometimes significantly. Even recent food and drink intake can nudge a reading up or down. People with higher muscle mass tend to have a higher resting metabolic rate, which generates more heat, while those with less muscle or lower overall metabolic activity tend to run slightly cooler.
The upshot is that there’s no single “normal” temperature that applies to everyone. A healthy adult’s baseline can fall anywhere from about 97.0°F to 99.0°F and still be perfectly fine. What matters most is knowing your own pattern so you can recognize when something has changed.