The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) figure most of us grew up hearing. That old number dates back to 1868, and modern research shows human body temperature has been slowly dropping ever since. Your own “normal” can vary by nearly two degrees depending on the time of day, your age, and how you measure it.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Reinhold August Wunderlich, who published a landmark study in 1868 analyzing 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 for more than 150 years.
The problem is that Wunderlich used armpit (axillary) readings, which tend to run lower than oral readings, and his thermometers were far less precise than what we use today. More importantly, humans themselves appear to have changed.
Why Body Temperature Has Dropped
A large Stanford University study published in eLife examined temperature records spanning nearly two centuries of American adults. The findings were clear: mean body temperature has decreased by about 0.03°C (roughly 0.05°F) per decade of birth. Men born in the early 1800s ran temperatures about 1.06°F higher than men today. Women showed a similar rate of decline, dropping about 0.58°F since the 1890s.
The reasons aren’t entirely settled, but the leading explanations involve reduced chronic inflammation. In the 19th century, tuberculosis, gum disease, and untreated infections were far more common, all of which keep the immune system active and body temperature elevated. Better sanitation, antibiotics, and dental care have lowered the baseline level of inflammation in the average person’s body, and with it, their resting temperature.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily cycle. You’re coolest in the early morning, roughly two hours before you wake up (typically between 4 and 6 a.m.), and warmest in the early evening. The difference between your daily low and high ranges from 0.5 to 1.9°F in healthy people. This means a reading of 99°F at 6 p.m. can be perfectly normal even if 97.5°F was your number that morning.
How Measurement Method Matters
The number on your thermometer depends heavily on where you take the reading. Rectal temperatures run highest and are considered the most accurate reflection of core body temperature. Oral readings come in slightly lower. Armpit readings are the least accurate and typically the lowest of the three.
These differences are reflected in fever thresholds. The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. But the specific cutoff shifts by method:
- Rectal, ear, or forehead: 100.4°F (38°C) or higher
- Oral: 100°F (37.8°C) or higher
- Armpit: 99°F (37.2°C) or higher
If you’re using an armpit thermometer and the result seems off, it’s worth rechecking with an oral or ear thermometer to confirm.
Age, Sex, and Hormones
Children tend to run warmer than adults, partly because their metabolisms are faster. Older adults tend to run cooler, which can make it harder to detect a fever in someone over 65. A temperature of 99°F in an elderly person may represent the same level of immune response as 101°F in a younger adult.
Hormonal cycles also shift the baseline. During the first half of the menstrual cycle (before ovulation), basal body temperature sits at its lower range. After ovulation, progesterone causes a rise of 0.5 to 1.0°F that lasts through the second half of the cycle. This shift is reliable enough that some people use daily temperature tracking as a fertility awareness method.
How Your Body Regulates Heat
Your core temperature stays remarkably stable even when the air around you changes dramatically. The body manages this through blood flow to the skin. When it’s hot, blood vessels near the surface dilate to release heat. When it’s cold, they constrict to keep warmth near your organs. As long as these adjustments can keep pace with the environment, your core temperature holds steady.
That system has limits, though. When the body can no longer shed heat fast enough, core temperature climbs, which is how heat stroke develops. On the other end, prolonged cold exposure overwhelms the body’s ability to conserve heat. Hypothermia begins when core temperature drops below 95°F (35°C), a point where shivering intensifies and mental confusion can set in.
What Your Reading Actually Tells You
Rather than comparing yourself to any single “normal” number, it’s more useful to know your own baseline. Take your temperature a few times when you feel healthy, at different times of day, and you’ll get a sense of your personal range. For most adults, that range falls somewhere between 97°F and 99°F when measured orally.
A reading that’s 1.5 to 2°F above your personal baseline is a more meaningful signal than crossing some fixed threshold. Someone who normally runs at 97.4°F might feel genuinely sick at 99.5°F, well before hitting the 100.4°F clinical fever mark. Context matters more than the number alone: how you feel, what time of day it is, and whether you’ve just exercised, eaten, or been out in extreme weather all affect the reading.