A narrow target
In a healthy adult the temperature deep inside the body, the core, is usually held near 37 degrees Celsius (about 98.6 degrees Fahrenheit). The figure is an average, not a fixed value. Normal readings vary somewhat between individuals, with the time of day and with the measurement site, and typically sit within a degree or so of that mark.
The skin and the limbs are cooler and fluctuate much more. The body is therefore better pictured as a warm core wrapped in a cooler outer shell, with the shell acting as a buffer and as the main place where heat is gained or lost.
Why the range is so tight
Nearly all biochemical reactions in the body, including those driven by enzymes, depend on temperature. If the core drifts far from the usual range, enzyme activity, nerve signaling and the integrity of cell structures are affected. Severe deviations, whether too hot or too cold, can impair the brain and other organs and are medical emergencies. That is why the body invests heavily in regulating temperature.
The heat budget
Temperature at any moment reflects a balance between heat gained and heat lost.
Heat production comes mainly from metabolism, the chemical reactions that turn food into usable energy. Much of the energy released is simply heat, since no process is perfectly efficient. At rest, organs like the liver, brain and heart produce a large share. During exercise, working muscles can raise the body’s heat output considerably, which is why people warm up so quickly when they run. These transformations reflect the broader conversion between forms of energy.
Heat exchange with the environment occurs through four routes:
- Conduction: Direct transfer by contact, such as touching a cold metal rail. Water conducts heat away far more effectively than air, which is one reason cold water is so dangerous.
- Convection: Transfer by moving fluid. Air next to the skin warms up and rises or is swept away, and wind speeds this up, which is the basis of the wind-chill effect.
- Radiation: Every warm object emits infrared radiation. A person loses heat by radiating to cooler surroundings and gains it from the sun or hot surfaces.
- Evaporation: When liquid water changes to vapor it absorbs a large amount of energy, a consequence of the properties of water. Evaporating sweat takes heat from the skin.
In a cool room at rest, much of the loss is through radiation and convection. When surroundings are as warm as the skin or warmer, those routes stop working or reverse, and evaporation becomes the main way to shed heat.
The body’s thermostat
Temperature sensors are spread through the skin, deep tissues and the brain. Their signals feed into the hypothalamus, a small brain region that compares the temperature with a target (the set point) and activates responses to correct any difference. It is a classic example of negative feedback.
Responses to heat
- Vasodilation: Blood vessels in the skin widen, bringing warm blood close to the surface so more heat can escape.
- Sweating: Sweat glands release fluid onto the skin. It cools only if it evaporates. In humid air, evaporation slows, so sweating is less effective and heat feels harder to bear, which is the physics behind why humid heat feels worse.
- Behavior: Seeking shade, removing clothing and drinking fluid are often the most powerful responses.
Because sweating loses water and salts, prolonged heat exposure raises the risk of dehydration and heat illness. Periods of extreme weather make these risks more relevant.
Responses to cold
- Vasoconstriction: Skin vessels narrow, reducing flow to the surface and cutting heat loss. Fingers and toes cool first as a result.
- Shivering: Rapid, involuntary muscle contractions that burn fuel and generate heat.
- Other heat sources: Some people, particularly infants, have brown fat that produces heat directly. Hormonal changes can also modestly raise metabolism.
- Behavior: Putting on clothing, which traps still air, and moving to shelter are again powerful.
Fever is a change in set point
During many infections, immune signals raise the hypothalamic set point, so the body acts to conserve and generate heat until the new, higher target is reached. This is different from heat stroke, where heat gain overwhelms regulation. Fever is often considered part of the body’s defense against viruses and bacteria, although whether lowering it is helpful or harmful depends on the situation, and medical advice is best for individual cases.
Variation and limits
Infants, older adults and people with certain illnesses or medications can regulate temperature less well. Sleep timing influences it too: core temperature typically dips at night and rises by day, in step with the body’s sleep cycles. Acclimatization can improve tolerance to heat over days or weeks, mainly by making sweating start earlier and work more efficiently.
This is a general science overview and not medical or safety advice for any specific situation.