Normal body temperature for a healthy adult averages around 98.6°F (37°C), but the true range spans from about 97°F (36.1°C) to 99°F (37.2°C). That 98.6 number has been the standard since the 1800s, but modern research suggests most people actually run cooler than that, closer to 97.5°F (36.4°C).
Why 98.6°F May Be Outdated
The 98.6°F benchmark dates back to the mid-1800s, when a German physician named Carl Wunderlich measured armpit temperatures from about 25,000 people and calculated the average. That number stuck for over a century. But when researchers analyzed oral temperatures from more than 150,000 people recorded between 2007 and 2017, they found the average had gradually dropped by more than one degree over the past 160 years. A 2023 study of over 35,000 people landed on an average of 97.9°F.
Two main factors explain the shift. First, people today have lower metabolic rates on average. Your metabolism generates heat the same way a running engine does, and modern humans seem to produce less baseline heat, possibly due to changes in body composition and overall health. Second, chronic infections and inflammation were far more common in Wunderlich’s era. Tuberculosis, syphilis, and untreated gum disease all raise body temperature, and those conditions were widespread when the original measurements were taken.
What Controls Your Body Temperature
A small region at the base of your brain called the hypothalamus acts as your internal thermostat. It continuously monitors your core temperature and makes adjustments through the nervous system: widening blood vessels and triggering sweating when you’re too warm, or constricting blood vessels and producing shivering when you’re too cold. This process happens automatically, without any conscious effort on your part.
Why Your Temperature Changes Throughout the Day
Your body temperature isn’t static. It follows a predictable daily cycle tied to your circadian rhythm. Temperature typically starts rising during the last hours of sleep, just before you wake up, and reaches its peak sometime in the late afternoon or early evening. It then drops again at night as your body prepares for sleep. Most people also experience a small dip between 2 p.m. and 4 p.m., which may partly explain that familiar afternoon drowsiness.
Several other factors shift your temperature throughout the day:
- Physical activity: Exercise raises core temperature significantly, especially in hot or humid environments. Even moderate walking in warm conditions can push your temperature well above its resting baseline.
- Menstrual cycle: During the second half of the cycle (the luteal phase), core temperature rises by roughly 0.3 to 1.0°F compared to the first half. This is driven by hormonal changes that also delay sweating and reduce blood flow to the skin, making heat dissipation slightly less efficient.
- Age: Older adults don’t necessarily have a different baseline temperature, but their ability to regulate temperature weakens. Less fat beneath the skin makes it harder to retain warmth, and a reduced ability to sweat increases the risk of overheating. Notably, older adults may not develop a fever even when fighting an infection, which can make illnesses harder to detect based on temperature alone.
Where You Measure Matters
Not all thermometer placements give the same reading. Rectal and ear thermometers typically read about 1.0°F (0.6°C) higher than oral thermometers. Forehead (skin) thermometers tend to read about 1.0°F lower than oral readings. Armpit measurements are generally the least accurate of all methods.
For young children, a rectal reading is considered the most reliable. If you take a child’s temperature with a forehead or armpit thermometer and the result seems questionable, a rectal measurement is the best way to confirm. These differences also matter when interpreting fever thresholds, since what counts as a fever depends on where you’re measuring.
When Temperature Signals a Problem
Fever
Most healthcare providers define a fever as an oral temperature of 100.4°F (38°C) or higher. Readings between 99.5°F and 100.3°F are often considered a low-grade fever. For children, the threshold depends on the method: 100.4°F rectally or by ear, 100°F orally, or 99°F under the arm.
Fever itself is not an illness. It’s a sign that your immune system is actively responding to something, usually an infection. The hypothalamus essentially raises your body’s thermostat set point, which is why you feel chilled even though your temperature is elevated.
Hypothermia
On the other end of the spectrum, a core temperature below 95°F (35°C) is classified as hypothermia. Mild hypothermia (95 to 89.6°F) causes shivering, confusion, and impaired coordination. As temperature drops further into moderate territory (89.6 to 82.4°F), shivering may stop and consciousness fades. Severe hypothermia, below 82.4°F (28°C), can cause the body to appear lifeless, with barely detectable breathing and heart rate. Older adults are particularly vulnerable because of their reduced ability to sense and respond to cold.
What a “Normal” Reading Means for You
Given everything that influences body temperature, your personal normal may not be 98.6°F. It could sit comfortably at 97.4°F or 98.8°F and be perfectly healthy. What matters more than hitting one specific number is knowing your own baseline. If you take your temperature regularly at the same time of day, using the same method, you’ll develop a sense of what’s typical for your body. That makes it much easier to recognize when something is genuinely off.