How can we explain why we sweat more when we are ill?

Key points to remember.

Sweating related to illness is triggered by the hypothalamus and makes it possible to combat the pathogens responsible for the infection.
When the fever decreases, the evaporation of sweat at the surface of the epidermis helps to disperse the excess heat from the body caused by the illness.
It is important to stay well hydrated when you sweat a lot, so that the body does not become dehydrated.

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Why does illness make us sweat?

To understand why we sweat when we are ill, it helps to picture our body as a building equipped with a thermostat. This thermostat is located in the hypothalamus, a small region of the brain. Under normal conditions, it keeps our temperature at around 37°C. However, when a virus or a bacterium enters the body, the immune system releases substances called endogenous pyrogens, produced in particular by white blood cells. These are mainly prostaglandins and inflammatory cytokines, such as interleukin-1 and TNF-α.

These are the molecules that signal to the hypothalamus that the body temperature needs to increase.

By raising the temperature above normal, the body creates a hostile environment for pathogens, limiting their reproduction while boosting the efficiency of immune cells. Perspiration generally occurs during two distinct phases of the illness:

The rising phase of fever : When your internal thermostat is set to 39°C but your body is still at 37°C, you experience shivering. The body generates heat through muscle contractions and reduces heat loss through vasoconstriction. The skin then tends to become cooler.
The phase during which the fever subsides : When the infection recedes, or when the effect of an antipyretic medicine – that is, a drug used to combat feverish states, such as paracetamol – takes place, the hypothalamus lowers its set point to 37°C. The body then finds itself overheated relative to this new target and must eliminate the excess heat. This is done via sweating. Indeed, the sweat released on the surface of the skin absorbs body heat in order to pass from a liquid to a gaseous state, thereby allowing the internal temperature to fall back towards its normal value.

It should be noted that the amount of sweat produced depends on the balance between metabolic heat production and heat exchange with the environment. If you are under several blankets while your fever is going down, the sweat cannot evaporate effectively, which may prolong your discomfort. Sweating is therefore the visible sign that your body has successfully mobilised its resources and is now attempting to stabilise itself.

Profuse sweating when ill: which measures should you adopt?

Perspiration is a defence mechanism of the body that should not systematically be suppressed.

Except in cases of major discomfort or very high fever, where antipyretics may become necessary, allowing the sweat to flow enables the body to complete its healing cycle. By trying to artificially suppress this sweating, one risks disrupting natural thermal regulation and prolonging the state of fatigue. It is better to see sweat as a way of expelling the excess energy accumulated during the battle against the infection.

The first instinct to adopt is not to stop the sweating, but to replace the water that it carries away. Fever-related sweating is particularly rich in water and mineral salts, that is to say in electrolytes. To support your metabolism, drink small sips of water, vegetable broths or herbal teas throughout the day, even if you do not immediately feel the need. Dehydration is often the main cause of the fatigue and headaches that follow an illness. By staying hydrated, you help your kidneys to filter the metabolic waste products generated during the immune response.

≈ 1 litre per day

of sweat lost per day on average without physical activity.

≈ 2.5 litres per hour

of sweat lost per hour at most when the body is exposed to extreme heat conditions.

A common mistake when ill is to overdress or to pile on too many covers, whether to try to feel better or to “sweat it out”. However, if you wrap yourself in several layers of synthetic blankets, you prevent air from circulating and sweat from evaporating. Yet it is precisely this transition from liquid to gas that actually cools the body. To support sweating without hindering it, choose loose-fitting clothing made from natural fibres such as cotton. As soon as you feel drenched in sweat, do not hesitate to take a proper shower and change your clothes and bed linen. Sleeping in a damp environment is not only uncomfortable, it also does not help to bring down a fever any faster.

Note : It is also best to avoid “sweating it out” by doing sport. Forcing physical exertion on a body already engaged in an immune response divides its energy resources. Rather than speeding up recovery, exercise exhausts your body, increases the risk of dehydration and, ultimately, makes it much less effective at fighting the infection. The priority should remain rest, so that your immune system can function properly.