A normal body temperature for most adults today is around 97.9°F (36.6°C), not the 98.6°F (37°C) figure you probably grew up hearing. That long-standing number dates back to the 1800s, and research from Stanford Medicine shows human body temperature has been steadily dropping since then. The real “normal” also depends on your age, the time of day, where you measure, and your individual biology.
Why 98.6°F Is Outdated
The 98.6°F standard came from a German physician’s measurements in the mid-1800s. It was a reasonable average for that era, but it no longer reflects the modern human body. A large Stanford Medicine analysis found that average body temperature in the U.S. has dropped by about 0.05°F every decade since the 19th century. Men born in the early 1800s had temperatures roughly 1°F higher than men today. Women showed a similar rate of decline, about 0.6°F lower since the 1890s.
The most likely explanation is improved health and living conditions. In the 1800s, chronic infections, untreated dental disease, and widespread inflammation were the norm. Inflammation raises body temperature. As sanitation, medicine, and nutrition improved, baseline inflammation dropped, and so did average temperatures. Today’s normal range for adults falls between 97.3°F and 98.2°F, with the overall average sitting at 97.9°F.
How Temperature Shifts Throughout the Day
Your body temperature isn’t a fixed number. It follows a predictable daily cycle driven by your internal clock. Temperatures are lowest in the early morning hours, then begin rising during the last stretch of sleep, just before you wake up. They peak in the late afternoon or early evening, then gradually fall again as nighttime approaches. Most people also experience a small dip between 2 p.m. and 4 p.m., which partly explains that familiar afternoon drowsiness.
This daily swing can easily account for a full degree of variation. A reading of 97.4°F at 7 a.m. and 98.4°F at 5 p.m. could both be perfectly normal for the same person on the same day. If you’re checking your temperature because you feel unwell, keep the time of day in mind before assuming a slightly elevated reading means fever.
Where You Measure Matters
Different parts of the body give different readings, sometimes by a meaningful margin. Oral temperature (under the tongue) is the most common reference point, and the offsets from other methods are fairly consistent:
- 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 97.2°F and an ear reading of 98.8°F could represent the same core temperature. If you’re comparing your number to published fever thresholds, make sure you know which method those thresholds assume. Most clinical guidelines are based on oral readings.
Factors That Raise or Lower Your Baseline
Several biological factors shift where your personal “normal” sits. Older adults tend to run cooler than younger adults, which means a temperature that looks unremarkable on paper could actually represent a significant fever in someone elderly. Children and infants, on the other hand, tend to run slightly warmer.
Hormonal cycles also play a role. After ovulation, basal body temperature rises by 0.4 to 1°F and stays elevated through the second half of the menstrual cycle. This shift is reliable enough that some people use daily temperature tracking as a fertility awareness method. Physical activity, recent meals, and even the temperature of the room can nudge your reading up or down by small amounts.
The key takeaway from the Stanford research is that normal temperature is personal. Two healthy people can have baseline temperatures that differ by nearly a full degree. Tracking your own temperature when you feel well gives you a much more useful reference point than any population average.
When Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. This threshold applies to adults and is widely used in clinical settings, airports, and workplace screening. It’s based on oral measurement.
That said, the gap between a “normal” of 97.9°F and a fever of 100.4°F is more than two degrees. Temperatures in between, roughly 99 to 100.3°F, are sometimes called “low-grade fever,” though there’s no universal clinical definition for that range. If you normally run cool (say, 97.3°F), a reading of 99.5°F represents a bigger jump for your body than it would for someone who normally sits at 98.2°F.
When Temperature Drops Too Low
On the other end of the scale, a core temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia, between 90°F and 95°F, causes shivering, confusion, and poor coordination. Moderate hypothermia sets in below 90°F, where shivering may actually stop and heart rhythm can become irregular. Severe hypothermia, below about 82°F, is a medical emergency with a high risk of cardiac arrest.
Hypothermia doesn’t require extreme cold. Older adults, very young children, and people on certain medications can develop it in moderately cool indoor environments, especially if they’re wet or inactive for extended periods.