The normal human body temperature is closer to 97.9°F (36.6°C), not the 98.6°F (37°C) most of us learned growing up. That old number dates back to the 1800s, and research from Stanford Medicine shows that average body temperature in the U.S. has been dropping by about 0.05°F per decade since then. Today, healthy adults typically fall somewhere between 97.3°F and 98.2°F.
Why 98.6°F Is Outdated
The 98.6°F standard came from a German physician’s measurements in the mid-19th century. It stuck around for over 150 years, but large-scale studies now confirm that humans have literally cooled down. The most likely explanation is a population-wide decrease in chronic inflammation. In the 1800s, infections like tuberculosis and malaria were far more common, dental hygiene was poor, and living conditions drove persistent low-grade inflammation that kept body temperatures higher.
Modern improvements in sanitation, medical care, and standard of living have reduced that background inflammation. The widespread use of anti-inflammatory medications, including common painkillers, has likely contributed as well. Researchers interpret the cooling trend as a sign of a genuine decrease in metabolic rate across the population, not just a quirk of better thermometers.
Your Temperature Changes Throughout the Day
Body temperature isn’t a fixed number. It follows your circadian rhythm, rising and falling in a predictable pattern over 24 hours. It’s lowest in the early morning hours (often dipping below 97°F) and peaks in the late afternoon or early evening. Most people also experience a slight dip between 2 p.m. and 4 p.m., which partly explains that post-lunch drowsiness.
Your temperature starts climbing during the last hours of sleep, just before you wake up, and gradually decreases again at night as your body prepares for rest. This daily swing can span a full degree or more, which means a reading of 97.5°F in the morning and 98.5°F in the evening can both be perfectly normal for the same person.
Factors That Shift Your Baseline
Age is one of the biggest influences. Older adults tend to run cooler than younger people, sometimes significantly so. This means a temperature that looks “normal” on paper could actually represent a fever in someone who’s elderly. Children and infants, on the other hand, tend to run slightly warmer and are more reactive to infections, spiking temperatures quickly.
For people who menstruate, the menstrual cycle creates a reliable temperature shift. Before ovulation, basal temperature (your resting temperature first thing in the morning) typically sits between 96°F and 98°F. After ovulation, a rise in progesterone pushes it up by 0.4°F to 1°F, settling into a range of 97°F to 99°F. This shift is the basis for temperature-based fertility tracking.
Physical activity, stress, hot or cold environments, recent meals, and even the time since your last drink of water can nudge your reading up or down. Because of all this variability, your personal normal matters more than any population average.
Where You Measure Makes a Difference
Rectal readings are the most accurate reflection of core body temperature, but they’re obviously the least convenient for everyday use. Oral thermometers are the most common choice for adults. Armpit (axillary) readings tend to run lower than oral or rectal measurements, and forehead (temporal artery) readings can vary depending on technique and ambient temperature.
There’s no reliable formula for converting between measurement sites. Adding or subtracting a degree to “translate” an armpit reading into an oral equivalent doesn’t hold up consistently. The better approach is to always measure the same way so you can spot changes from your own baseline. If you normally take your temperature orally, stick with that.
When a Temperature Becomes a Fever
A fever is generally defined as a temperature of 100.4°F (38°C) or higher, whether measured orally, rectally, or with an ear or forehead thermometer. For armpit readings, the threshold is lower: 99°F (37.2°C) or above. Anything between your normal baseline and these cutoffs is sometimes called a low-grade fever, though it can also just reflect normal daily variation or recent physical activity.
Adults with temperatures reaching 103°F (39.4°C) or higher typically look and feel noticeably ill. On the other end of the spectrum, hypothermia begins when core temperature drops below 95°F (35°C), which impairs the body’s ability to function and requires prompt warming.
Because individual baselines vary, context matters. A reading of 99.5°F might be unremarkable for a young adult measured in the afternoon, but could signal a problem for an elderly person whose temperature normally hovers around 97°F. Knowing your own typical range gives you better information than relying on a single universal number.