A healthy body temperature for most adults falls between 97.0°F and 99.0°F (36.1°C to 37.2°C), not the 98.6°F you probably grew up hearing. That old number dates back to 1851, when a German physician averaged millions of readings from patients in Leipzig. Human body temperature has measurably dropped since then, and 98.6°F is now higher than what most healthy people register on a thermometer.
Why 98.6°F Is Outdated
The 98.6°F standard held for over 150 years, but a large Stanford University study published in eLife found that body temperatures have been falling steadily since the Industrial Revolution. Men born in the early 1800s ran about 1.06°F (0.59°C) warmer than men today, declining at a rate of roughly 0.05°F per decade. Women showed a similar pattern, dropping about 0.58°F (0.32°C) since the 1890s.
The reasons aren’t entirely clear, but the leading explanations center on reduced chronic inflammation. People in the 19th century lived with untreated infections, poor dental health, and unhealed wounds far more often than people in high-income countries do today. Widespread use of anti-inflammatory medications may also play a role. Whatever the cause, the shift is real: today’s average healthy adult simply runs cooler than their great-great-grandparents did.
Your Temperature Changes Throughout the Day
Your body doesn’t hold one steady temperature. It cycles through a roughly 1.8°F (1°C) swing every 24 hours, driven by your internal clock. You’re coolest in the early morning, usually between 4 and 6 a.m., when readings can dip to 96.8°F (36°C) or even lower. Your temperature peaks in the late afternoon or early evening, potentially reaching 100.3°F (37.9°C) rectally or 99.9°F (37.7°C) orally, both of which are completely normal.
This means a reading of 99.5°F at 5 p.m. might be perfectly fine, while that same number at 6 a.m. could signal something is off. Context matters more than any single cutoff.
Factors That Shift Your Baseline
Several things can push your temperature up or down without anything being wrong:
- Menstrual cycle: After ovulation, body temperature rises by 0.4°F to 1.0°F (0.22°C to 0.56°C) and stays elevated through the second half of the cycle. This is the principle behind basal body temperature tracking for fertility.
- Exercise: Intense physical activity can raise core temperature by 2°F to 5°F (1.2°C to 2.8°C). Cooling back down takes surprisingly long, roughly four to five times the duration of the workout. A 15-minute run, for example, can leave your temperature elevated for about 90 minutes.
- Age: Older adults tend to run cooler than younger adults, sometimes significantly so. A temperature that looks “normal” on paper might actually represent a fever in someone over 65. Newborns and young children, on the other hand, tend to run slightly warmer.
- Time of eating: Digesting food generates heat, which can bump your reading up slightly for an hour or two after a meal.
- Ambient temperature: Spending time in very hot or cold environments can temporarily shift your reading, especially when measured at the skin surface.
Where You Measure Matters
Different parts of the body give different readings, and the gaps are consistent enough that you can roughly convert between them. Compared to an oral (mouth) reading:
- Rectal and ear: Read 0.5°F to 1.0°F (0.3°C to 0.6°C) higher
- Armpit and forehead: Read 0.5°F to 1.0°F (0.3°C to 0.6°C) lower
So if your forehead thermometer says 98.0°F, your actual oral temperature is closer to 98.5°F to 99.0°F. This is especially important for children: rectal readings are considered the most accurate for infants and toddlers. For older children and adults, oral thermometers are the most practical balance of accuracy and convenience. Armpit readings are the least reliable but work fine for quick screening.
When a Temperature Becomes a Fever
The CDC defines a fever as a measured temperature of 100.4°F (38°C) or higher. Below that but above your personal normal is sometimes called a low-grade fever, roughly 99.0°F to 100.3°F, though this isn’t a formal clinical category.
For young children, the thresholds look similar. A rectal temperature of 100.4°F or above in a baby under 3 months old is treated more urgently than the same reading in an older child. In adults over 65, a lower temperature, say 99.0°F to 100.0°F, may already represent a meaningful fever because their baseline tends to be lower.
When a Temperature Is Too Low
On the other end of the scale, a core body temperature below 95°F (35°C) is classified as hypothermia. The stages break down by severity:
- Mild (90°F to 95°F): Shivering, confusion, poor coordination
- Moderate (82°F to 90°F): Shivering may stop, drowsiness increases, heart rate slows
- Severe (below 82°F): Loss of consciousness, dangerously slow heart rate
Hypothermia is most commonly associated with cold weather exposure, but it can also happen indoors in elderly adults or people with certain medical conditions who can’t regulate their temperature well.
Finding Your Personal Normal
Because “normal” spans a range and varies by person, the most useful thing you can do is learn your own baseline. Take your temperature a few times over several days when you feel well, using the same thermometer and the same method each time. Measure at roughly the same time of day, ideally mid-morning or mid-afternoon to avoid the natural extremes. After a few readings, you’ll have a personal average you can compare against when you feel sick.
For most people, that number will land somewhere between 97.0°F and 98.8°F orally. If yours sits a bit outside that range but is consistent and you feel fine, it’s simply your normal. The number on the thermometer matters less than the change from your baseline.