How does the body regulate temperature? Sweating, shivering and fever

TUREFF• •0

Whether you step out into a frosty morning or sit in a hot bath, the temperature deep inside your body barely changes. Keeping that core temperature within a narrow range is one of the body's constant background jobs, because the chemistry inside our cells works best within it. This guide explains how the system works, what fever really is, and when heat or cold become dangerous.

The thermostat in the brain

A small region at the base of the brain called the hypothalamus acts as the body's thermostat. It receives information from temperature sensors in the skin and inside the body, compares it with a set point, and triggers responses to warm up or cool down. Normal temperature is not one fixed number. It varies a little from person to person, through the day, with activity and, for women, across the menstrual cycle. It is often quoted as roughly 37C, but a reading somewhat above or below that can be perfectly normal.

How the body cools down

When you are getting too warm, the body has two main tools.

Widening blood vessels in the skin. More warm blood flows close to the surface, where heat can escape to the air. This is why you look flushed when hot.
Sweating. Sweat on the skin evaporates, and evaporation carries heat away. This works well in dry air but much less well in humid conditions, when sweat drips off rather than evaporating, which is why muggy heat feels so oppressive.
Behaviour helps too: seeking shade, removing layers, drinking and slowing down.

How the body warms up

When you are getting too cold, the response runs the other way.

Narrowing blood vessels in the skin keeps warm blood towards the core, protecting vital organs. Hands and feet feel cold first for this reason.
Shivering is rapid, involuntary muscle contraction. Muscles produce heat as they work, and shivering turns them into a heater.
Goosebumps are a leftover from furrier ancestors, whose raised hair trapped a layer of insulating air. In humans they do very little.
Brown fat, a type of fat tissue that burns energy to make heat, plays a part in babies and to a lesser extent in adults.

What fever is

A fever is not the thermostat breaking. It is the thermostat being deliberately turned up, usually in response to an infection.

Immune signals reach the hypothalamus and raise the set point.
Your body then feels cold at a normal temperature, so you shiver, feel chilly and want to wrap up until your temperature reaches the new set point. That is why the start of a fever often comes with shivers.
When the fever breaks, the set point drops back, and the body now feels too hot, so you sweat and throw off the covers.
A higher temperature is thought to help the immune system and make conditions harder for some germs, which may be why fever evolved.

In the UK, the NHS usually treats a temperature of 38C or above as a high temperature, also called a fever, though you may feel hot or shivery even below that.

When heat and cold become dangerous

Heat exhaustion can bring heavy sweating, headache, dizziness, nausea and cramps. Moving somewhere cool, lying down, drinking water and cooling the skin usually help.
Heatstroke is a medical emergency, when the body can no longer cool itself. Warning signs include confusion and feeling unwell even after cooling down. Get emergency help.
Hypothermia happens when the body loses heat faster than it can make it. Signs include intense shivering, then confusion and drowsiness. It is also an emergency.
Older people, babies and young children are more vulnerable to both extremes.

When to see a doctor

This guide is general information. For a fever, follow your national health service's advice, and get medical help if you are worried, the fever lasts several days, or it comes with symptoms such as a rash, a stiff neck, difficulty breathing or confusion. Babies and young children with a high temperature have their own guidance, so check it or ask a doctor. Curious about why you feel cold before a fever, or anything else here? Ask in the comments.