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 famous figure dates back to the 1860s and, while it was accurate for its time, human body temperature has been steadily dropping since then. Your own “normal” can vary depending on the time of day, your age, how you measure it, and even where you are in your menstrual cycle.
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 analyzing over one million armpit temperature readings from roughly 25,000 patients. It was an enormous dataset for its era, and his conclusion that 37°C (98.6°F) represented the upper limit of normal body temperature became medical gospel for more than 150 years.
The problem is that humans have gotten cooler since then. A large study published in eLife, using data spanning nearly two centuries, found that body temperature in the United States has dropped by about 0.03°C (0.05°F) per decade of birth. Men born in the early 1800s ran temperatures roughly 0.59°C (1.06°F) higher than men today. Women showed a similar pattern, with a decline of about 0.32°C (0.58°F) since the 1890s. Overall, modern humans in high-income countries run about 1.6% cooler than people in the pre-industrial era.
Why Humans Are Running Cooler
The leading explanation is that chronic inflammation has decreased dramatically. In the 1800s, infections like tuberculosis, syphilis, and periodontal disease were far more common and often went untreated. Persistent infection raises baseline body temperature. With modern sanitation, antibiotics, and dental care, our immune systems are fighting fewer constant low-level battles, which brings the average down. Climate-controlled environments may also play a role: when your body doesn’t have to work as hard to stay warm or cool, its resting metabolic rate can settle lower.
How Your Body Controls Its Temperature
A small region in your brain called the hypothalamus acts as your internal thermostat. It constantly receives signals about your current body temperature and adjusts heat production or heat loss accordingly. When you’re too warm, specialized neurons suppress heat-generating activity throughout your body, causing you to sweat and send blood toward your skin to radiate heat away. When you’re too cold, a different set of neurons ramps up thermogenesis, triggering shivering and constricting blood vessels near the surface to conserve warmth. This system keeps your core temperature remarkably stable even as the air around you swings by dozens of degrees.
Normal Fluctuations Throughout the Day
Your body temperature isn’t a fixed number. It follows a predictable daily rhythm, hitting its lowest point in the early morning (around 5 to 7 AM, roughly two hours before you typically wake up) and peaking in the evening (around 8 to 10 PM, about two hours before you fall asleep). The swing between your daily low and high is typically 0.4 to 1.4°F (0.2 to 0.8°C). This means a reading of 97.5°F at 6 AM and 98.5°F at 9 PM could both be perfectly normal for the same person on the same day.
Age, Sex, and Individual Differences
Infants and young children tend to run slightly warmer than adults because their metabolic rate is higher relative to their body size. Older adults tend to run cooler. In a study of 92 healthy adults aged 64 and older, the average oral temperature was 98.3°F (36.86°C), already below the traditional 98.6°F benchmark.
In younger adults, women often run slightly warmer than men, partly because of hormonal fluctuations tied to the menstrual cycle. After ovulation, during the luteal phase, basal body temperature rises by at least 0.4°F (0.22°C) and stays elevated until menstruation begins. This shift is small but consistent enough that some people use it to track ovulation. In older adults, this sex-based difference largely disappears once menstrual cycles end.
Race can also factor in. In the same study of older adults, Black women had average temperatures about 0.23°F (0.13°C) higher than white women of the same age. The difference was statistically significant, though small in practical terms.
How Measurement Method Affects Your Reading
The number on your thermometer depends heavily on where you take it. Oral readings (under the tongue) are the most common reference point for adults. Compared to an oral reading:
- Rectal temperatures read 0.5 to 1.0°F (0.3 to 0.6°C) higher
- Ear (tympanic) temperatures read 0.5 to 1.0°F (0.3 to 0.6°C) higher
- Armpit (axillary) temperatures read 0.5 to 1.0°F (0.3 to 0.6°C) lower
So if your oral temperature is 98.0°F, an armpit reading might show 97.2°F and a rectal reading might show 98.7°F, all representing the same actual body temperature. Rectal readings are considered the most accurate, which is why they’re standard for infants. For adults, oral thermometers are accurate enough for everyday use as long as you haven’t eaten, drunk anything, or smoked in the previous 15 minutes.
When a Temperature Becomes a Fever
An oral temperature of 100°F (37.8°C) or higher is generally considered a fever. That threshold hasn’t changed much despite the shift in baseline averages, because fever represents the body intentionally raising its set point to fight infection, not just a random fluctuation. A temperature of 103°F (39.4°C) or higher warrants medical attention.
Knowing your own baseline matters here. If you typically run at 97.5°F, a reading of 99.5°F represents a two-degree jump, which your body will feel like a fever even though it technically falls below the clinical threshold. Paying attention to your normal range over time gives you a better sense of when something is off than relying on a single cutoff number.