The widely cited “normal” body temperature of 98.6 °F (37.0 °C) is outdated. A landmark 1992 study at the University of Maryland found that the actual average oral temperature in healthy adults was closer to 98.2 °F (36.8 °C), with an upper limit of normal around 99.9 °F rather than the traditional 100.4 °F cutoff.1PubMed Central. A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich More recent data suggest the number has continued drifting downward. Your “normal” depends on the time of day, where you take the measurement, your age, your sex, and even what decade you were born in.
Where the 98.6 Standard Came From
The 98.6 °F figure traces back to a German physician named Carl Wunderlich, who published it in the 1860s after reportedly collecting millions of armpit temperature readings from thousands of patients. For over a century, his number went unchallenged. But when researchers at the University of Maryland revisited the question using oral thermometers in 148 healthy adults, they found the mean was about half a degree Fahrenheit lower than Wunderlich claimed. They also confirmed that temperature hits its low point around 6 a.m. and peaks between 4 and 6 p.m., with an average daily swing of about 0.9 °F. Women ran slightly warmer than men, and there was a trend toward higher readings in Black participants compared to white participants.1PubMed Central. A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich
Part of the discrepancy is likely measurement site: Wunderlich used armpit readings, which tend to read lower than oral ones, yet somehow arrived at a higher average. His instruments were also less precise than modern thermometers, and his patient population included people who may not have been entirely healthy. Whatever the explanation, 98.6 °F stuck in medical textbooks and public consciousness long after the evidence moved on.
Humans Are Getting Cooler Over Time
Even the updated 98.2 °F average may already be a bit high for people born in recent decades. A 2020 analysis of three large datasets spanning nearly 200 years of American health records found a steady decline in body temperature across birth cohorts. For men, the drop was about 0.03 °C per decade, adding up to roughly 0.59 °C between the early 1800s and the late 1990s. For women, the decline was similar in rate but measured over a shorter historical window.2PubMed Central. Decreasing human body temperature in the United States since the Industrial Revolution That means someone born today likely runs a little cooler than their great-grandparents did at the same age.
The trend is not confined to wealthy, industrialized countries. A study of the Tsimane, an indigenous population in Bolivia with high rates of infection, found a comparable decline in body temperature over just two decades, suggesting the cooling trend may not be driven entirely by reduced infectious disease burden. Reductions in chronic inflammation, changes in ambient temperature thanks to climate-controlled environments, and lower resting metabolic rates have all been proposed as contributors, though no single explanation has been proven.3PubMed Central. Rapidly declining body temperature in a tropical human population
How Your Temperature Shifts Through the Day
Your body temperature is not a fixed number. It follows a roughly 24-hour cycle tied to your circadian rhythm, bottoming out in the early morning and climbing through the afternoon. A large-scale analysis of wearable device data found the daily minimum between 6 and 8 a.m. and the maximum between 6 and 8 p.m.4PubMed. The daily, weekly, and seasonal cycles of body temperature analyzed at large scale The swing can be close to a full degree Fahrenheit in either direction from your personal average.
This has real consequences for detecting fever. An emergency department study found that the proportion of patients arriving with elevated temperatures was about two and a half times higher in the evening than in the morning.5PubMed Central. Fever Incidence Is Much Lower in the Morning than the Evening: Boston and US National Triage Data It is not that more people get sick at night; it is that the body’s natural temperature peak in the late afternoon and evening means a person fighting off an infection is more likely to cross the fever threshold during those hours. If you take your temperature first thing in the morning and see 99.5 °F, that is a more meaningful signal than the same number at 7 p.m.
Where You Measure Makes a Bigger Difference Than You Might Think
The readings you get from different body sites can vary by more than a degree. In an emergency department study comparing oral, rectal, and tympanic (ear) measurements taken from the same patients, the mean oral temperature was 98.3 °F, while mean rectal and tympanic temperatures were 99.4 °F and 99.6 °F, respectively. But the averages hide a wide spread: individual oral readings could be as much as 2.9 °F lower than the corresponding rectal measurement in the same person.6PubMed Central. Oral and Tympanic Membrane Temperatures Are Inaccurate to Identify Fever in Emergency Department Adults
A systematic review pooling data from studies with strong evidence found the following normal ranges in healthy adults:
- Oral: 33.2–38.2 °C (roughly 91.8–100.8 °F)
- Rectal: 34.4–37.8 °C (roughly 93.9–100.0 °F)
- Tympanic: 35.4–37.8 °C (roughly 95.7–100.0 °F)
- Axillary (armpit): 35.5–37.0 °C (roughly 95.9–98.6 °F)
The review also noted that men and women have somewhat different ranges, with women showing a wider spread for oral readings.7PubMed. Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review Rectal temperature is generally treated as the closest approximation to core body temperature in clinical settings, which is why many fever thresholds are technically set against rectal readings. When your doctor says “fever starts at 100.4 °F,” they usually mean a rectal value.
Not All Thermometers Are Created Equal
The type of thermometer you use adds yet another layer of variability. During the COVID-19 pandemic, contactless forehead thermometers became ubiquitous at building entrances and clinics, but their accuracy leaves a lot to be desired. In one study at an African outpatient clinic, a contactless infrared thermometer detected only about 13% of fevers that were confirmed by oral thermometry. By contrast, a temporal artery thermometer (the kind you swipe across the forehead) performed better, with roughly 88% sensitivity and 88% specificity.8PubMed Central. Are all thermometers equal? A study of three infrared thermometers to detect fever in an African outpatient clinic
A systematic review and meta-analysis comparing digital, infrared, and mercury-in-glass thermometers against rectal readings found that infrared devices (both temporal and tympanic) were generally good at confirming the absence of fever but not especially good at catching it. In practical terms, a sub-100.4 °F reading from a peripheral device does not necessarily rule out a true fever.9PubMed Central. The diagnostic accuracy of digital, infrared and mercury-in-glass thermometers in measuring body temperature: a systematic review and network meta-analysis Another study comparing several infrared devices against a non-invasive core temperature sensor found poor agreement across the board, with temple and nose measurements showing almost no correlation with core readings.10PubMed Central. Clinical accuracy of infrared temperature measurement devices: a comparison against non-invasive core-body temperature If you are using a contactless forehead gun at home, treat it as a rough screening tool rather than a precise measurement.
Age Changes What “Normal” Looks Like
Older adults tend to run cooler than younger ones, and this shift has clinical consequences that go beyond semantics. A systematic review confirmed that healthy elderly people have lower average body temperatures across all common measurement sites compared with younger adults. The decline is thought to stem from a slower metabolic rate combined with a reduced ability to adjust to environmental temperature changes.11Open Forum Infectious Diseases. Normal Body Temperature: A Systematic Review
The more worrisome implication is that older adults may fail to mount a detectable fever even when seriously infected. Research estimates that fever is absent or blunted in roughly 20 to 30% of elderly patients with infections.12Clinical Infectious Diseases. Fever in the Elderly Common use of acetaminophen and similar medications in this age group can further dampen the temperature response, compounding the problem.13PubMed Central. Approach to Low Body Temperature or Mild Hypothermia in the Geriatric Population: A Narrative Review For caregivers and clinicians, this means a reading of 99 °F in an 85-year-old who normally sits at 97.2 °F could be just as significant as 101 °F in a younger person. A shift from someone’s personal baseline matters more than any single threshold number.
At the other end of the age spectrum, infants tend to run warmer. One pediatric study found that both axillary and tympanic temperatures were statistically higher in children aged 0 to 2 months compared with older age groups.14Pediatric Emergency Care. Axillary and Tympanic Temperature Measurement in Children and Normal Values for Ages Babies also have less developed thermoregulation, which is why pediatricians treat fever in very young infants more urgently than they would in older children.
The Menstrual Cycle and Body Temperature
If you have ever tracked your basal body temperature for fertility purposes, you already know that hormone levels reshape the thermal landscape across the menstrual cycle. Core body temperature rises by about 0.3 to 0.7 °C after ovulation, during the luteal phase, when progesterone levels are elevated.15PubMed Central. Temperature regulation in women: Effects of the menstrual cycle This is consistent enough to serve as the basis for natural family planning methods and has been validated by wearable skin temperature sensors that track the same pattern nightly.16npj Women’s Health. Understanding wrist skin temperature changes to hormone variations across the menstrual cycle
The relationship between progesterone and temperature is not perfectly linear, though. A study tracking thousands of menstrual cycles found a moderate correlation between progesterone metabolite levels and basal body temperature. Temperature plateaued once progesterone levels rose above a certain concentration, meaning further increases in the hormone did not keep pushing temperature higher.17PubMed. Descriptive analysis of the relationship between progesterone and basal body temperature across the menstrual cycle For practical purposes, the post-ovulatory thermal shift is a reliable signal, but its magnitude varies from cycle to cycle and person to person.
Exercise, Heat, and Temporary Extremes
Physical exertion can push core temperature well above any resting “normal.” During prolonged exercise, especially in warm conditions, sweating and rising internal heat production are both natural responses.18PubMed. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies In a study of recreational runners completing a 10 km treadmill run under heat stress, average core temperature at the finish reached about 39.6 °C (103.3 °F).19PubMed Central. Predicting the body core temperature of recreational athletes at the end of a 10 km self-paced run under environmental heat stress That would alarm anyone checking a thermometer in a resting context, but for an athlete mid-effort it is within the expected range and usually resolves within an hour of stopping.
Environmental temperature itself nudges your baseline even at rest. A study of older adults found that ambient temperature and sex were the strongest predictors of oral temperature, with people aged 85 and above being the most susceptible to environmental influence.20PubMed. Normal body temperature and the effects of age, sex, ambient temperature and body mass index on normal oral temperature: a prospective, comparative study And outdoor temperature affects even internal hormonal rhythms: a large dataset of basal body temperature readings from women showed a significant association between external temperature and body temperature in both the follicular and luteal phases of the menstrual cycle.21PubMed Central. Age-Dependent and Seasonal Changes in Menstrual Cycle Length and Body Temperature Based on Big Data
How Your Brain Manages the Thermostat
The region of the brain that orchestrates all of this is the preoptic area of the hypothalamus, a small cluster of neurons near the base of the brain. These neurons are directly sensitive to changes in blood temperature and also receive signals from temperature sensors in the skin and spinal cord.22PubMed. Role of the preoptic-anterior hypothalamus in thermoregulation and fever By integrating information from both internal and external sources, this area decides whether to trigger heat-generating responses (like shivering) or heat-shedding responses (like sweating and dilating blood vessels near the skin).
Animal research has drilled down further into specific cell types. In mice, activating a particular set of inhibitory neurons in this region reliably dropped body temperature, while blocking those same neurons caused fever-like hyperthermia, demonstrating that the system actively holds temperature in check rather than just producing heat on demand.23PubMed Central. A hypothalamic circuit that controls body temperature The preoptic area also plays a role in coordinating sleep, which helps explain why your lowest body temperature aligns with the deepest part of your sleep cycle.24PubMed Central. Role of the Preoptic Area in Sleep and Thermoregulation
When you are cold, two main heat-generating systems kick in. Shivering is the most obvious: involuntary muscle contractions that produce warmth quickly. The other is non-shivering thermogenesis, driven largely by brown adipose tissue, a specialized fat that burns calories to generate heat rather than storing energy.25PubMed Central. Brown fat thermogenesis and cold adaptation in humans How much each system contributes has been debated. Recent work using a skin-temperature clamping technique found that shivering increased in a predictable, dose-dependent way as skin got colder, while fat-based heat production did not scale up as linearly.26PubMed. Shivering, but not adipose tissue thermogenesis, increases as a function of mean skin temperature in cold-exposed men and women In other words, when you are truly chilled, it is mainly your muscles doing the heavy lifting to warm you back up.
Medications That Move the Dial
Certain drugs can quietly shift your resting temperature or impair your body’s ability to cool itself. A systematic review and meta-analysis examining prescription and over-the-counter medications during heat stress found that drugs with strong anticholinergic properties raised core temperature by about 0.4 °C under warm conditions, alongside reduced sweating. Non-selective beta-blockers and anti-Parkinson’s agents also produced small but measurable elevations.27PubMed Central. The effect of prescription and over-the-counter medications on core temperature in adults during heat stress: a systematic review and meta-analysis Antidepressants and diuretics, somewhat surprisingly, did not alter core temperature responses in the pooled analysis. For people taking these medications during heat waves, the risk is less about running a persistent low-grade fever and more about losing the ability to shed heat efficiently when the environment is already hot.
Shift Work and Circadian Disruption
If your work schedule puts you on the night shift, your temperature rhythm gets dragged out of alignment with the rest of your physiology. A study of hospital nurses found that night-shift workers showed a greater gap between the time they were awake and the time their core body temperature hit its daily minimum. They also had more fragmented sleep and performed worse on cognitive tests at the end of their shifts, even after accounting for where they were in their temperature cycle.28PubMed Central. Misaligned core body temperature rhythms impact cognitive performance of hospital shift work nurses This suggests that the temperature rhythm is not just a passive byproduct of your daily schedule but is functionally linked to alertness and performance. When the rhythm is misaligned, the consequences extend well beyond feeling groggy.
Why Knowing Your Own Baseline Matters
Given the sheer number of variables that influence body temperature, a single universal cutoff for “fever” is a blunt tool. Researchers have begun making the case for precision approaches that interpret temperature against an individual’s habitual range rather than a population average. One study tracking continuous temperature and oxygen saturation data argued that deviations from a person’s own normal interval can flag health changes earlier than applying a standard threshold to everyone.29Medical Research Archives. Interpretation of Body Temperature and Oxygen Saturation based on Individual Normal Ranges in Habitual Condition: A Step Forward for Applied Precision Medicine
You do not need a wearable sensor to benefit from this thinking. Taking your temperature at the same time of day, using the same device and the same body site, over a couple of weeks when you are healthy gives you a rough personal baseline. A thermometer reading that is a full degree above that number is more informative than comparing a single measurement to an arbitrary population average that was calculated from 19th-century armpit readings.
Body Composition and Thermal Comfort
Your build influences how you experience temperature, though not always in the direction people assume. Research examining the interplay of fat and muscle on thermal preference found that individuals with higher body fat or higher muscle mass both tended to prefer cooler environments, while people with lower fat and lower muscle mass leaned toward warmer ones.30Building and Environment. Effects of body muscle and fat on differences in thermal preference The insulating effect of fat is real, but muscle generates substantial metabolic heat at rest, so a lean, muscular person and an overweight, less muscular person can end up with surprisingly similar thermal comfort preferences for entirely different physiological reasons. If you have always run hot or cold compared to the people around you, your body composition is likely part of the explanation, alongside the hormonal and circadian factors already discussed.