The average human body temperature is about 97.9°F (36.6°C), not the 98.6°F (37°C) number most of us grew up hearing. That familiar figure dates back to the 1860s and, while it was a reasonable estimate for its time, modern research shows human body temperature has been gradually declining for over a century. Your own “normal” can also shift throughout the day, vary by age and sex, and read differently depending on where you place the thermometer.
Where 98.6°F Came From
The 98.6°F standard traces back to a German physician named Carl Wunderlich, who published a landmark study in 1868 based on over one million armpit temperature readings from roughly 25,000 patients. It was an impressive effort for the era, but the thermometers he used were bulky instruments that required 15 to 20 minutes under the arm just to stabilize. Armpit readings also tend to run lower than oral or rectal measurements, introducing another source of error. Still, Wunderlich’s number became medical gospel and stuck around for more than 150 years.
Why the Average Has Dropped
A large Stanford University analysis published in 2020 examined temperature records spanning nearly two centuries and found that average body temperature in the United States has been falling steadily, about 0.03°C (roughly 0.05°F) per decade of birth. Men born in the early 1800s ran temperatures about 0.59°C (1.06°F) higher than men today. Women showed a similar rate of decline, dropping about 0.32°C (0.58°F) since the 1890s.
The reasons aren’t fully settled, but the leading explanations point to reduced rates of chronic infection (thanks to antibiotics, better sanitation, and vaccines), lower levels of background inflammation, and climate-controlled living environments. In short, our bodies may simply be doing less immunological work at rest than they were a century ago.
What’s Actually Normal Today
Rather than a single number, a healthy resting body temperature typically falls within a range of 97°F to 99°F (36.1°C to 37.2°C) when measured orally. The midpoint sits closer to 97.9°F (36.6°C) based on modern data, with a 95% confidence interval running from about 96.3°F to 99.1°F (35.7°C to 37.3°C). Where you fall in that range depends on the time of day, your age, your sex, and even your menstrual cycle phase.
Fever is generally defined as 100.4°F (38°C) or higher when measured rectally, in the ear, or at the temple. For an oral reading, 100°F (37.8°C) is the usual threshold. For armpit readings, 99°F (37.2°C) is considered elevated.
Your Temperature Changes Throughout the Day
Body temperature follows a predictable daily rhythm tied to your circadian clock. It bottoms out in the early morning, roughly two hours before you wake up, typically between 5 and 7 AM. It then climbs gradually and peaks in the evening, around 8 to 10 PM, about two hours before you fall asleep. The total swing from lowest to highest point ranges from about 0.4°F to 1.4°F (0.2°C to 0.8°C), depending on the person.
This means a reading of 97.5°F at 6 AM and 98.9°F at 9 PM could both be perfectly normal for the same person. If you’re tracking your temperature for any reason, taking it at the same time each day gives you the most useful comparison.
Where You Measure Matters
The number on your thermometer depends heavily on where you place it. Rectal readings come closest to true core body temperature. Oral readings run slightly lower. Armpit readings are the least accurate and tend to underestimate core temperature by the widest margin. Ear (tympanic) and forehead (temporal artery) thermometers fall somewhere in between and are convenient but can be thrown off by technique or ambient conditions.
If an armpit reading seems off, it’s worth confirming with an oral or rectal measurement. This is especially true for young children, where an accurate reading can determine whether a fever needs attention.
Age, Sex, and the Menstrual Cycle
Older adults tend to run cooler than younger people, which can make it harder to detect fevers in people over 65. A temperature of 99°F in an elderly person may represent a more significant immune response than the same reading in a 30-year-old.
In people who menstruate, basal body temperature rises slightly after ovulation, typically by less than half a degree Fahrenheit (about 0.3°C). This small but consistent shift is the basis of temperature-based fertility tracking. The rise occurs because of hormonal changes during the second half of the menstrual cycle and persists until the next period begins (or continues if pregnancy occurs).
Exercise and Heat Exposure
Physical activity raises core temperature substantially. During vigorous exercise in hot conditions, well-trained athletes can reach internal temperatures as high as 106.7°F (41.5°C) without suffering harm. For most people doing moderate exercise, core temperature rises a degree or two above baseline and returns to normal within 30 to 60 minutes of stopping. This is a normal part of how the body generates and dissipates heat during physical work, not a sign of illness.
Taking your temperature immediately after a workout, a hot shower, or time in direct sun will give you an artificially high reading. Wait at least 20 minutes in a comfortable environment for a more representative number.