Body Temperature: Normal Range, Regulation & When to Worry

What Is Normal Body Temperature?

Normal body temperature for both adults and children typically ranges from 36 to 37.8°C (96.8 to 100°F). This "core" temperature refers to the internal temperature of vital organs, while skin temperature varies more widely depending on environmental conditions and blood flow.

The concept of a single "normal" body temperature has evolved significantly since German physician Carl Reinhold August Wunderlich established 37°C (98.6°F) as the standard in 1851 after measuring temperatures in over 25,000 patients. Modern research has shown that this figure represents an average, and healthy individuals can have baseline temperatures that fall anywhere within a broader range.

Your internal or "core" temperature remains relatively stable because your vital organs – particularly the brain, heart, and liver – require consistent conditions to function optimally. The enzymes that drive virtually every chemical reaction in your body work most efficiently at specific temperatures, which is why your thermoregulatory system works continuously to maintain this narrow range.

Skin temperature, in contrast, can vary dramatically. When you're cold, blood vessels near the skin constrict to conserve heat, causing skin temperature to drop. When you're warm, blood flow to the skin increases, and you may notice flushing as your body attempts to release excess heat. This explains why touching someone's forehead gives only a rough indication of internal temperature.

Factors That Affect Your Baseline Temperature

Several factors influence what's "normal" for any given individual, making it useful to know your own baseline when healthy:

• Time of day: Temperature follows a circadian rhythm, dropping during sleep and reaching its lowest point around 4-6 AM, then rising throughout the day to peak around 4-6 PM. This variation can be as much as 0.5-1°C (0.9-1.8°F).
• Age: Older adults, particularly those with dementia or who require significant daily assistance, often have lower baseline temperatures. Newborns and young children may have slightly higher normal temperatures.
• Physical activity: Exercise generates heat as a byproduct of muscle metabolism. Vigorous activity can temporarily raise core temperature by 1-2°C.
• Menstrual cycle: Women who ovulate experience a temperature rise of approximately 0.3-0.5°C after ovulation that persists until menstruation. After menopause, baseline temperature typically decreases slightly.
• Pregnancy: Elevated progesterone levels maintain higher body temperatures throughout pregnancy.
• Thyroid function: Hyperthyroidism (overactive thyroid) gradually increases body temperature as thyroid hormones accelerate metabolism. Hypothyroidism has the opposite effect.
• Stress and anxiety: Emotional stress increases metabolic rate, which can elevate body temperature. This is sometimes called "psychogenic fever."

How Does Your Body Regulate Temperature?

Body temperature is regulated by the hypothalamus, a small region in the brain that acts as your internal thermostat. Temperature sensors throughout your body send signals to the hypothalamus, which then activates warming mechanisms (shivering, vasoconstriction) or cooling mechanisms (sweating, vasodilation) to maintain optimal temperature.

The hypothalamus functions remarkably like a thermostat in your home, but with far greater precision and adaptability. This walnut-sized structure at the base of your brain receives constant input from two types of temperature sensors: central thermoreceptors that monitor blood temperature directly, and peripheral thermoreceptors located in your skin that detect environmental temperature changes.

When the hypothalamus detects that your core temperature has deviated from its set point, it initiates a cascade of physiological responses through both the nervous system and hormonal signals. These responses fall into two categories: those that generate or conserve heat when you're cold, and those that dissipate heat when you're warm.

The sophistication of this system becomes apparent when you consider how precisely it works. Your body can detect temperature changes of less than 0.5°C and respond appropriately, maintaining core temperature within a narrow range despite enormous variations in environmental conditions – from arctic cold to tropical heat.

How Your Body Produces Heat

When your core temperature drops below the set point, several mechanisms activate to generate and conserve heat:

Shivering thermogenesis is the most obvious heat-generating response. When you feel cold, your muscles begin rhythmic contractions that you experience as shivering. This involuntary movement converts chemical energy to heat with remarkable efficiency – shivering can increase heat production by up to five times the resting rate. The sensation of "chattering teeth" and general muscle tension that precedes visible shivering represents the early stages of this response.

Non-shivering thermogenesis occurs primarily through metabolic activity in brown adipose tissue (brown fat). Unlike regular white fat, which stores energy, brown fat contains abundant mitochondria that can "waste" energy as heat. Infants have particularly high amounts of brown fat because they cannot shiver effectively. Adults retain some brown fat, primarily around the neck and upper back, though the amount decreases with age and obesity.

Hormonal responses also contribute to heat production. The thyroid gland can increase production of thyroid hormones, which elevate metabolic rate throughout the body. The adrenal glands release epinephrine (adrenaline), which stimulates metabolic activity. These hormonal changes occur more gradually than shivering but can sustain elevated heat production for longer periods.

Vasoconstriction conserves heat by reducing blood flow to the skin. When blood vessels near the body's surface narrow, less warm blood reaches areas where heat would be lost to the environment. This explains why your hands, feet, and nose become pale and cold when you're chilled – blood is being redirected to keep your vital organs warm.

How Your Body Loses Heat

When core temperature rises above the set point, the body activates cooling mechanisms:

Vasodilation is the opposite of vasoconstriction. Blood vessels near the skin surface expand, bringing more warm blood to the skin where heat can dissipate into the environment. This causes the flushing or redness you notice when you're hot or after exercise. Increased blood flow to the skin can significantly accelerate heat loss, particularly in cooler environments or with air movement.

Sweating is the body's most powerful cooling mechanism. Sweat glands throughout your skin release a watery solution onto the skin surface. As this moisture evaporates, it absorbs heat from the skin, cooling the blood flowing just beneath. Evaporative cooling is highly effective – the evaporation of one liter of sweat removes approximately 580 kilocalories of heat energy. However, sweating only works when the sweat can evaporate; high humidity reduces its effectiveness significantly.

Behavioral adaptations complement these physiological responses. While not directly controlled by the hypothalamus, the sensation of being too hot or cold motivates behaviors like seeking shade, drinking cold fluids, removing clothing, or stopping physical activity – all of which help restore normal temperature.

What Causes Fever and When Should You Worry?

Fever (temperature ≥38°C/100.4°F) is your body's deliberate response to infection, not a malfunction. When immune cells detect pathogens, they release chemicals called pyrogens that signal the hypothalamus to raise the temperature set point. While uncomfortable, fever helps fight infection and is rarely dangerous in itself.

Understanding fever requires a shift in perspective: fever is not your body losing control of temperature regulation, but rather your body intentionally resetting its thermostat to a higher level. This elevated temperature serves several purposes in fighting infection. Many bacteria and viruses replicate less efficiently at higher temperatures. Additionally, elevated temperature enhances immune cell activity, speeds antibody production, and increases the delivery of nutrients to sites of infection.

The process begins when your immune system detects invaders. White blood cells and other immune cells release signaling molecules called pyrogens – both the body's own (endogenous) pyrogens like interleukin-1 and interleukin-6, and pyrogens from the invading organisms themselves (exogenous). These pyrogens travel through the bloodstream to the hypothalamus, where they trigger the release of prostaglandins that raise the temperature set point.

When the set point rises, your body perceives its current temperature as "too cold" even though it hasn't changed. This triggers heat-generating responses: you feel chilled, your skin becomes pale as blood vessels constrict, and you begin to shiver. Once your core temperature rises to match the new set point, these symptoms stop. When the fever breaks – either because the infection is resolving or because of fever-reducing medication – the set point drops back to normal. Now your body perceives its elevated temperature as "too hot," triggering sweating and flushing as it releases the excess heat.

Fever Severity and What the Numbers Mean

While the height of a fever doesn't always correlate with the severity of illness, general categories help guide appropriate responses:

• Low-grade fever (38-38.9°C / 100.4-102°F): Common with minor viral infections. Usually not concerning in otherwise healthy adults and children over 3 months. May not require treatment.
• Moderate fever (39-39.9°C / 102.2-103.8°F): Indicates more significant immune response. Consider fever-reducing medication if uncomfortable. Monitor for other symptoms.
• High fever (40-41°C / 104-105.8°F): Requires attention. While still usually the body responding appropriately to infection, seek medical evaluation, especially in young children or if accompanied by concerning symptoms.
• Hyperpyrexia (>41°C / >105.8°F): Rare and potentially dangerous. Requires immediate medical attention. May indicate overwhelming infection, hypothalamic dysfunction, or drug reaction.

Should You Treat a Fever?

The decision to treat fever depends more on how you feel than on the temperature reading itself. Since fever serves a protective function, reducing it may not speed recovery and might even prolong some infections. However, fever-reducing medications (antipyretics) like acetaminophen (paracetamol) and ibuprofen can provide significant comfort and are appropriate when fever causes distress.

Reasons to consider treating fever include: significant discomfort, difficulty sleeping, reduced fluid intake due to feeling unwell, or history of febrile seizures in children. For most healthy adults and older children, letting a moderate fever run its course while staying hydrated and resting is a reasonable approach.

What Happens When Body Temperature Gets Too Low?

Hypothermia occurs when core body temperature drops below 35°C (95°F), a condition that can be life-threatening. As temperature falls, metabolic processes slow, including heart rate and brain function. Without treatment, severe hypothermia (below 28°C/82°F) can cause cardiac arrest. Hypothermia requires immediate medical attention and controlled rewarming.

While fever represents the body deliberately raising temperature, hypothermia is the failure of thermoregulation to maintain adequate warmth. This typically occurs when heat loss exceeds heat production for an extended period, overwhelming the body's compensatory mechanisms. Unlike fever, hypothermia is always a medical concern that requires intervention.

The progression of hypothermia follows a predictable pattern as core temperature drops. In mild hypothermia (32-35°C / 89.6-95°F), intense shivering occurs as the body attempts to generate heat. Mental function begins to decline, with confusion and poor judgment common. Victims may not recognize they're in danger, which is particularly problematic because their impaired judgment prevents them from seeking help or shelter.

As temperature continues to fall into the moderate range (28-32°C / 82.4-89.6°F), shivering paradoxically stops as the body's energy reserves deplete and muscle function deteriorates. Heart rate and breathing slow significantly. Victims become increasingly drowsy and confused, and coordination deteriorates markedly. At this stage, victims may engage in "paradoxical undressing" – removing clothing despite being cold, possibly due to blood vessel dilation that temporarily creates a sensation of warmth.

Severe hypothermia (below 28°C / 82.4°F) is immediately life-threatening. The heart becomes extremely susceptible to dangerous arrhythmias, and cardiac arrest is common. Breathing may become so slow and shallow as to be barely detectable. In extreme cases, victims may appear dead, with no detectable pulse or breathing, yet can sometimes be successfully resuscitated – hence the medical principle "you're not dead until you're warm and dead."

Controlled Hypothermia in Medicine

Interestingly, the same properties that make hypothermia dangerous can be medically useful. During certain surgeries – particularly heart and brain operations – physicians intentionally cool patients to reduce their metabolic needs. When tissues require less oxygen and nutrients, surgeons have more time to complete complex procedures without causing damage. The cooling must be carefully controlled and the rewarming equally precise to prevent complications.

What Happens When Body Temperature Gets Too High?

Hyperthermia (heat-related illness) occurs when the body absorbs or generates more heat than it can dissipate, causing core temperature to rise above 40°C (104°F). Unlike fever, hyperthermia is not controlled by the hypothalamus and can quickly become life-threatening. Heat stroke is a medical emergency requiring immediate cooling.

It's crucial to distinguish hyperthermia from fever. In fever, the hypothalamus deliberately raises the temperature set point as part of the immune response. In hyperthermia, the thermoregulatory system is either overwhelmed by environmental heat or prevented from cooling properly, and the body's temperature rises beyond the set point against the hypothalamus's "wishes."

Heat exhaustion is the earlier, less severe form of heat-related illness. It occurs when the body struggles to cool itself, typically through heavy sweating that leads to dehydration and salt loss. Symptoms include heavy sweating, weakness, nausea, headache, and dizziness. Core temperature may be elevated but typically remains below 40°C (104°F). Heat exhaustion is serious but usually responds to rest in a cool environment, fluid replacement, and removal of excess clothing.

Heat stroke represents a medical emergency. Core temperature exceeds 40°C (104°F), and the thermoregulatory system fails. The skin may be hot and dry (classic heat stroke) or still moist (exertional heat stroke in athletes or workers). Mental status changes are a hallmark – confusion, combativeness, or unconsciousness. Without rapid cooling, heat stroke can cause organ damage and death within hours. Treatment focuses on rapid cooling using ice water immersion when possible, combined with aggressive fluid resuscitation.

Children are particularly vulnerable to heat-related illness because their thermoregulatory systems are less developed. They have a higher surface-area-to-volume ratio, causing faster heat absorption from the environment. Children also rely more heavily on sweating, which depletes fluids more quickly. Additionally, children may not recognize or communicate symptoms of overheating effectively.

How Do You Accurately Measure Body Temperature?

For the most accurate temperature reading, rectal measurement remains the gold standard for core temperature, particularly in infants. Oral thermometers are convenient and reliable for older children and adults. Ear (tympanic) and forehead (temporal) thermometers offer speed and convenience but may be less precise. Wait 15-30 minutes after eating, drinking, or exercise for accurate oral readings.

The method you use to measure temperature affects both the accuracy of the reading and how to interpret the result. Different sites reflect core temperature with varying degrees of accuracy, and normal ranges differ between measurement locations.

Rectal temperature provides the closest approximation of core body temperature and remains the recommended method for infants under 3 months and whenever accuracy is critical. Normal rectal temperature is typically 0.5°C (0.9°F) higher than oral temperature. While accurate, this method is less practical and comfortable for routine use in older children and adults.

Oral temperature is the most common method for adults and older children. For accuracy, the thermometer should be placed under the tongue with lips closed, avoiding recent hot or cold beverages (wait 15-30 minutes) and breathing through the mouth. Normal oral temperature ranges from 36-37.8°C (96.8-100°F).

Ear (tympanic) temperature measures infrared heat from the eardrum, which shares blood supply with the hypothalamus. Results are quick (1-2 seconds) but can be affected by earwax, ear infections, or improper probe placement. Readings are typically close to oral temperature. These thermometers may be less accurate in children under 6 months.

Forehead (temporal artery) temperature measures infrared heat from the temporal artery. Non-invasive and quick, these thermometers have become popular but may be affected by ambient temperature, sweating, or hair. They're useful for screening but may require confirmation with another method if fever is suspected.

Axillary (armpit) temperature is the least accurate method but may be appropriate for screening. Readings are typically 0.5-1°C (0.9-1.8°F) lower than oral temperature. The thermometer should remain in place for several minutes for accurate reading.

How Does Temperature Regulation Differ in Special Groups?

Infants, elderly adults, and people with certain medical conditions have less effective thermoregulation and face higher risks from temperature extremes. Newborns cannot shiver effectively and rely on brown fat for heat production. Elderly individuals may have blunted temperature responses and lower baseline temperatures, making fever less apparent even during serious infections.

Understanding how different groups regulate temperature helps recognize when intervention is needed and what constitutes normal variation for that population.

Temperature Regulation in Infants

Newborns face unique thermoregulatory challenges. Their large surface-area-to-volume ratio causes rapid heat loss to the environment. They cannot shiver effectively, instead relying primarily on metabolic heat production from brown adipose tissue (brown fat). This specialized fat, located around the neck, chest, and kidneys, generates heat through a process called non-shivering thermogenesis.

Because of these limitations, newborns are particularly vulnerable to hypothermia, especially in the first few hours after birth. Hospitals take extensive measures to keep newborns warm, including skin-to-skin contact with parents, warming beds, and careful temperature monitoring. At home, parents should maintain room temperature around 20-22°C (68-72°F) and dress infants in one more layer than adults would wear comfortably.

Conversely, any fever in an infant under 3 months requires immediate medical evaluation. Young infants' immature immune systems may not mount obvious responses to infection, and fever may be the only sign of serious bacterial illness.

Temperature Regulation in Older Adults

Aging affects thermoregulation in multiple ways. Elderly individuals often have lower baseline temperatures, reduced ability to perceive temperature changes, diminished sweating capacity, and less effective vasoconstriction and vasodilation. Certain medications common in older adults (beta-blockers, sedatives) can further impair thermoregulation.

These changes have important implications. Older adults may develop serious infections without typical fever, making diagnosis more challenging – instead of fever, they might show confusion, weakness, or falls. They're also more susceptible to both hypothermia and hyperthermia, even in conditions that younger adults would tolerate easily.