Thermoregulation Basics: How the Body Handles Heat and Cold

What Thermoregulation Means

Thermoregulation is the body's way of keeping its core temperature relatively constant. A physiology review states that humans normally maintain a body temperature of about 37 °C, and that this control is central to survival. [1] The core is the temperature deep in the body, as opposed to the skin, which can be much warmer or cooler.

The idea behind most hydrotherapy safety advice is simple. Hot water pushes the body toward gaining heat, cold water pushes it toward losing heat, and the body's control system can be overwhelmed if the push is strong, long or sudden, or if the system is not working well.

How the Body Responds to Heat and Cold

The same review lists the body's tools: widening and narrowing of blood vessels (vasodilation and vasoconstriction), sweating, heat produced without shivering, goose bumps, shivering, and changes in behavior such as getting out of the water. [1] A larger review of heat stress describes temperature regulation as a feedback system that keeps deep body temperature within narrow limits despite wide changes in surroundings and activity. [2]

In water, these responses are mixed with the effects of pressure and buoyancy. In cold water, a physiology review reports that skin and muscle blood vessels narrow and maximal cardiac output falls. [3] See how water affects the body.

What Affects How Well the Body Copes

The heat-stress review describes body size and build, heat adaptation, biological sex and age as factors that influence deep body temperature. It also discusses neurological, cardiovascular, metabolic and genetic diseases, and injuries such as spinal cord injury, deep burns and heat stroke. [2] In practice, this means two people in the same tub can have quite different responses. Medicines can matter as well: a 2026 review found that safety margins for heat therapy in frail older adults and in people taking beta-blockers remain insufficiently defined. [4]

This is why this library gives no single temperature or time that is right for everyone, and why people with heart disease, blood pressure problems, diabetes or reduced skin sensation, multiple sclerosis or other heat-sensitive conditions, kidney disease, or who are pregnant or older, should ask a clinician first. See preparing for hydrotherapy.

When Regulation Is Overwhelmed

Overheating. The NHS lists heat exhaustion signs such as excessive sweating with pale, clammy skin and cramps, and says heatstroke is a medical emergency needing immediate help. [5] In the US, call 911 for suspected heatstroke. See heat illness and overheating.

Getting too cold. The NHS defines hypothermia as a dangerous drop in body temperature below 35 °C, where normal is around 37 °C, and calls it a medical emergency. [6] A separate danger comes first in cold water: the cold shock response begins within seconds of immersion, before the core cools. [7] See cold shock, hypothermia and afterdrop.

Pregnancy. A 2006 review of hot tub and spa use in pregnancy described an elevated core temperature of 2 °C above baseline or more, and the timing and duration of exposure, as the critical factors for risk. [8] It also noted that hot tub temperature readings can vary. [8] See hot tubs, saunas and hot baths in pregnancy.

Heat, Cold and Where They Are Covered

Warm baths, hot tubs and sitz baths are covered in this library. Sauna, infrared sauna and cold plunge evidence sits with the biohacking pages: sauna: evidence summary, cold-water immersion: evidence summary and temperature exposure safety.

Evidence Labels for This Topic

The basic physiology of temperature regulation is established in textbooks and reviews. Claims that deliberate heat or cold "train" the body or improve health are a separate question, covered in heat and cold adaptation.