A normal body temperature for most adults is closer to 97.9°F (36.6°C), not the 98.6°F (37°C) number you probably grew up hearing. That familiar standard dates back to 1868, and large-scale research now shows that human body temperature has been steadily dropping since then. Your personal normal also shifts throughout the day, varies by age and sex, and depends on where you take the measurement.
Why 98.6°F Is Outdated
The 98.6°F benchmark comes from a German physician named Carl Wunderlich, who published over a million temperature readings from roughly 25,000 patients in the mid-1800s. His thermometers were bulky instruments that took 15 to 20 minutes to stabilize, and he measured under the arm rather than in the mouth. Both factors likely skewed his results compared to modern readings.
But the real shift goes beyond measurement tools. A study published in eLife, analyzing data spanning nearly two centuries, found that average body temperature has dropped by about 0.03°F per decade of birth. Men born in the early 1800s ran temperatures roughly 1°F higher than men today. Women have seen a similar decline of about 0.6°F since the 1890s. Altogether, the average human body temperature in high-income countries is about 1.6% lower than it was before the Industrial Revolution.
Researchers at Stanford Medicine pinpointed the current average more precisely. After analyzing over 618,000 oral temperature readings from adult outpatients between 2008 and 2017, they found normal adult temperatures ranging from 97.3°F to 98.2°F, with an overall average of 97.9°F. Julie Parsonnet, the lead researcher, has noted that using a single number like 98.6°F as a cutoff creates a false line between “normal” and “not normal” that doesn’t reflect how temperature actually works in real people.
Why Temperatures Have Dropped Over Time
The leading explanation is a broad reduction in chronic inflammation across the population. In the 1800s, infections like tuberculosis and malaria were widespread, dental hygiene was poor, and war injuries left many people with lingering inflammatory conditions. All of that raises baseline body temperature. As sanitation improved, antibiotics became available, and chronic infections declined, the low-level inflammation that once kept temperatures elevated faded with them.
Modern use of anti-inflammatory medications, including common over-the-counter painkillers, likely contributes as well. There’s also a simpler factor: climate control. People today spend far more time in temperature-regulated environments than they did two centuries ago. When your body doesn’t have to work as hard to stay warm or cool down, its resting metabolic rate drops, and temperature follows.
What Changes Your Temperature Throughout the Day
Your body temperature isn’t a fixed number. It follows a predictable daily rhythm driven by your internal clock. Temperatures are coolest in the early morning hours, begin rising during the last stretch of sleep just before you wake up, and peak around 4 p.m. Most people also experience a small dip between 2 p.m. and 4 p.m., which may partly explain that familiar afternoon drowsiness. The total swing from your daily low to your daily high can be around 1°F or more, which means a reading of 97.5°F at 7 a.m. and 98.5°F at 5 p.m. could both be perfectly normal for the same person.
The Stanford analysis found that time of day was the single biggest factor influencing a person’s temperature reading, more than age, sex, height, or weight. So if you’re checking your temperature and want a meaningful comparison, try to measure at a consistent time each day.
How Age, Sex, and Weight Factor In
Several personal characteristics shift your baseline. Men tend to run slightly cooler than women. Temperature decreases with age, which is why older adults often register readings that seem low. It also decreases slightly with height and increases with weight. Together, age, sex, height, weight, and time of day account for about 25% of the variation in any individual’s normal temperature and about 7% of the variation between different people.
The age effect is especially relevant for older adults. Because their baselines run lower, a temperature that looks unremarkable on a thermometer might actually represent a significant increase for them. A reading of 99°F in an 80-year-old could signal the same immune response that would show up as 100.5°F in a younger person.
Hormones and Physical Activity
For premenopausal women, body temperature shifts predictably across the menstrual cycle. During the luteal phase (the roughly two weeks after ovulation), hormonal changes raise baseline temperature. This is the principle behind temperature-based fertility tracking: a sustained rise of about 0.5°F to 1°F after ovulation signals that it has occurred. Hormonal contraceptives can alter this pattern as well.
Exercise raises core temperature as a natural consequence of muscle activity and heat production. Prolonged or intense exercise, particularly in hot conditions, can push core temperature well above the normal range without indicating illness. This is why a post-workout temperature reading isn’t a reliable baseline. For the most accurate picture of your resting temperature, measure before physical activity or at least 30 minutes after cooling down.
Measurement Method Matters
Where you place the thermometer changes the number you get. Rectal readings are closest to true core body temperature and run the highest. Oral readings are slightly lower. Armpit (axillary) readings are the least accurate and tend to be the lowest, sometimes by a full degree or more compared to rectal measurements. Ear (tympanic) thermometers fall somewhere in between but can be thrown off by earwax or a curved ear canal.
These differences are reflected in fever thresholds. For adults and children, a fever is generally defined as:
- 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 comparing readings over time, stick with the same method and the same thermometer. Mixing methods makes it nearly impossible to spot meaningful trends.
When a Low Temperature Is a Concern
Just as high temperatures signal fever, abnormally low temperatures can indicate a problem. Hypothermia begins when core body temperature drops below 95°F (35°C). At that point the body starts losing heat faster than it can produce it. Mild hypothermia (95°F to 89.6°F) causes shivering, confusion, and difficulty with coordination. Moderate hypothermia (89.6°F to 82.4°F) brings worsening confusion, slurred speech, and a dangerous slowing of heart rate. Severe hypothermia, below 82.4°F, is life-threatening.
Outside of cold exposure, a persistently low temperature can sometimes reflect an underactive thyroid, severe infection (particularly in older adults, where the body may not mount a fever response), or certain metabolic conditions. If your resting temperature consistently reads below 97°F and you’re experiencing fatigue or other symptoms, it’s worth investigating.
Finding Your Personal Normal
Rather than comparing yourself to a universal number, it helps to establish your own baseline. Take your temperature at the same time of day, using the same method, over the course of a week or two when you’re feeling well. The average of those readings is a far better reference point than 98.6°F. Once you know your personal normal, a deviation of 1°F or more above that baseline is a more reliable signal that something is off than hitting any arbitrary threshold on a chart.