A Quick Guide to Body Temperature

"We've been measuring body temperature for over 150 years, but we're only now beginning to understand how much information it actually contains. Temperature is a window into your circadian clock, your thyroid function, your immune system, and your metabolic rate—all from a single number."

— Dr. Samuel Armitage, Front Door Labs

Body temperature is arguably the most familiar vital sign. Most people know that 98.6°F (37°C) is considered "normal" and that a fever means something is wrong. But the reality is far more nuanced than a single number. Your body temperature fluctuates throughout the day, varies by measurement site, changes with age, and provides a surprising amount of information about your overall health when tracked consistently.

What Is "Normal" Body Temperature?

The widely cited 98.6°F (37°C) standard dates back to 1851, when German physician Carl Reinhold August Wunderlich measured axillary (armpit) temperatures in over 25,000 patients. However, modern research has consistently shown that average human body temperature has actually declined over the past century and a half.

A large 2020 study from Stanford University, analyzing data from over 677,000 temperature measurements spanning 157 years, found that average body temperature in the United States has dropped by approximately 0.05°F per decade since the 1860s. The current average oral body temperature for a healthy adult is closer to 97.5–97.9°F (36.4–36.6°C).

Normal Ranges by Measurement Site

• Oral (mouth): 97.6–99.6°F (36.4–37.6°C)
• Rectal: 98.2–100.2°F (36.8–37.9°C)—typically 0.5–1°F higher than oral
• Axillary (armpit): 96.6–98.6°F (35.9–37°C)—typically 0.5–1°F lower than oral
• Tympanic (ear): 97.6–99.6°F (36.4–37.6°C)—similar to oral
• Temporal (forehead): 97.2–99.2°F (36.2–37.3°C)—convenient but less precise

It's important to understand that there is no single "correct" body temperature. A range of 1–2°F around an individual's personal baseline is considered normal variation. Knowing your own baseline is more clinically useful than comparing to a population average.

What Affects Body Temperature?

Numerous factors influence body temperature at any given moment. Understanding these variables helps distinguish normal fluctuations from clinically significant changes.

• Time of day: Temperature follows a circadian pattern, lowest in early morning and highest in late afternoon (see below)
• Physical activity: Exercise can raise core temperature by 2–3°F or more
• Food and drink: Hot beverages, spicy food, and the thermic effect of eating can temporarily elevate temperature
• Hormones: Progesterone raises basal temperature by 0.4–0.8°F after ovulation; thyroid hormones are major regulators of metabolic heat production
• Age: Infants and young children run slightly warmer; older adults tend to have lower baseline temperatures and a blunted fever response
• Ambient environment: Extreme heat or cold, humidity, and clothing layers all influence surface temperature
• Medications: Beta-blockers, NSAIDs, acetaminophen, and some psychiatric medications can alter temperature regulation
• Hydration status: Dehydration impairs the body's ability to dissipate heat, raising core temperature

The Circadian Temperature Rhythm

One of the most fascinating aspects of body temperature is its predictable daily rhythm, driven by the suprachiasmatic nucleus—the same master clock that governs sleep-wake cycles, hormone release, and dozens of other biological processes.

Core body temperature typically reaches its lowest point (the "nadir") between 3:00 and 5:00 a.m., often dropping to 96.8–97.4°F. It then rises steadily through the morning, peaks between 4:00 and 7:00 p.m. at roughly 99.0–99.5°F, and declines again through the evening as the body prepares for sleep. This temperature drop in the evening is one of the key signals that promotes sleep onset—which is why a cool bedroom (65–68°F) supports better sleep.

Disruptions to this circadian temperature rhythm—from shift work, jet lag, irregular sleep schedules, or excessive evening light exposure—are associated with poor sleep quality, metabolic dysfunction, and increased disease risk.

Fever: When Temperature Signals Infection

Fever is the body's deliberate elevation of its thermoregulatory set point in response to infection, inflammation, or other immune challenges. It's not a malfunction—it's an evolved defense mechanism. Elevated temperature enhances immune cell function, inhibits pathogen replication, and accelerates healing.

Fever Classification

• Low-grade fever: 99.1–100.4°F (37.3–38°C)—may indicate mild infection, inflammation, or post-vaccination response
• Moderate fever: 100.4–102.2°F (38–39°C)—common with viral and bacterial infections
• High fever: 102.2–104°F (39–40°C)—requires monitoring and often treatment
• Hyperpyrexia: Above 104°F (40°C)—medical emergency requiring immediate intervention

Current clinical guidance generally recommends against aggressively treating low-grade fevers in otherwise healthy adults, as fever-reducing medications may slow immune clearance of infection. However, fever management is appropriate when the patient is uncomfortable, at risk of febrile seizures (particularly children), or has a fever above 102°F that persists for more than 48 hours.

Hypothermia: When the Body Gets Too Cold

Hypothermia occurs when core body temperature drops below 95°F (35°C). It's classified by severity.

• Mild hypothermia (90–95°F / 32–35°C): Shivering, confusion, impaired coordination, slurred speech
• Moderate hypothermia (82–90°F / 28–32°C): Shivering stops, severe confusion, drowsiness, paradoxical undressing
• Severe hypothermia (below 82°F / 28°C): Loss of consciousness, cardiac arrhythmias, risk of cardiac arrest

Hypothermia can occur not only from environmental cold exposure but also from certain medical conditions (hypothyroidism, sepsis, hypoglycemia), medications (general anesthesia, sedatives), and in elderly individuals with reduced thermoregulatory capacity. It's a medical emergency that requires gradual rewarming under medical supervision.

Basal Body Temperature Tracking

Basal body temperature (BBT) is your resting temperature measured immediately upon waking, before any activity. BBT tracking has two primary clinical applications.

Fertility and Ovulation Tracking

After ovulation, rising progesterone levels cause a sustained BBT increase of 0.4–0.8°F that persists through the luteal phase until menstruation (or continues if pregnancy occurs). By charting BBT daily, individuals can confirm ovulation, identify their fertile window retrospectively, detect luteal phase defects (short or insufficient temperature rise), and monitor early pregnancy (sustained elevated BBT beyond the expected period date).

Thyroid Function Assessment

Dr. Broda Barnes popularized BBT as a screening tool for hypothyroidism in the 1940s. His reasoning was sound: thyroid hormones are primary regulators of metabolic rate and heat production. Consistently low BBT readings (below 97.4°F / 36.3°C oral) can suggest subclinical hypothyroidism, even when standard TSH levels appear normal. While BBT alone is insufficient for diagnosis, it's a useful data point that many functional medicine practitioners incorporate into thyroid assessments.

Wearable Temperature Monitoring

The rise of wearable health technology has made continuous temperature monitoring accessible and practical. Devices like the Oura Ring, WHOOP strap, Apple Watch, and dedicated temperature patches (such as TempDrop for fertility tracking) measure skin temperature throughout the day and night, providing trend data that was previously impossible to capture.

What Wearables Can Detect

• Illness onset: Elevated nighttime temperature trends often appear 1–2 days before symptoms manifest, providing an early warning signal
• Ovulation: Continuous temperature monitoring provides more reliable ovulation confirmation than single morning measurements
• Recovery status: Post-exercise temperature recovery patterns indicate readiness for training
• Circadian alignment: Normal temperature curves confirm healthy circadian function; flat or inverted curves suggest disruption
• Stress and overtraining: Chronically elevated nighttime temperatures may indicate systemic inflammation, overtraining, or inadequate recovery

It's worth noting that wearable devices measure skin temperature, not core temperature. Skin temperature tracks core temperature trends but is typically 3–5°F lower and more susceptible to environmental influence. The clinical value lies in trend analysis over days and weeks, not in any single reading.

Temperature and Metabolic Health

Body temperature is fundamentally a reflection of metabolic rate. Every biochemical reaction in your body produces heat as a byproduct, and your resting temperature represents the sum of that metabolic activity. This is why temperature trends can serve as a proxy for metabolic health.

The historical decline in average body temperature (the Stanford study noted a drop of about 0.5°F since the 1860s) has been attributed to several factors: reduced chronic infection rates (thanks to improved sanitation and antibiotics), more stable ambient temperatures (central heating and cooling), and potentially reduced metabolic rates associated with modern sedentary lifestyles and changes in body composition.

From a clinical perspective, chronically low body temperature warrants investigation. It may indicate hypothyroidism or subclinical thyroid dysfunction, chronic stress with adrenal insufficiency, caloric restriction or nutritional deficiency, chronic fatigue syndrome or fibromyalgia, or iron deficiency anemia. Conversely, chronically elevated temperature (outside the normal circadian pattern) may signal hidden infection or chronic inflammation, autoimmune activity, or hyperthyroidism.

"I encourage every patient to track their temperature for at least two weeks. It's free, it's non-invasive, and the patterns it reveals—circadian rhythm health, thyroid function, immune status, recovery capacity—provide clinical insights that no single blood test can match. Temperature is the original biomarker, and it deserves a comeback."

— Dr. Samuel Armitage, Front Door Labs