How can we explain that we sweat more when we are sick?

Key points to remember.

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

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Why does illness make you sweat?

To understand why we sweat when we are sick, 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 body temperature around 37°C (98.6°F). However, when a virus or bacterium enters the body, the immune system releases substances called endogenous pyrogens, mainly produced by white blood cells. These are primarily 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 temperature above its normal level, the body creates a hostile environment for pathogens, limiting their reproduction while boosting the effectiveness of immune cells. Sweating generally occurs during two distinct phases of the illness:

The fever escalation phase : When your internal thermostat is set to 39°C but your body is still at 37°C, you experience chills. The body generates heat through muscle contractions and reduces heat loss through vasoconstriction. As a result, the skin tends to become cooler.
The fever drop phase : When the infection subsides or when an antipyretic medication—that is, a drug used to combat fever, such as acetaminophen—takes effect, the hypothalamus lowers its set point back to 37°C (98.6°F). The body then finds itself overheated relative to this new target and must eliminate the excess heat. This is done via perspiration. In fact, the sweat released on the surface of the skin absorbs body heat in order to change from a liquid to a gaseous state, which allows the internal temperature to return toward its normal value.

Note 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 coming down, the sweat cannot evaporate efficiently, which can prolong your discomfort. Sweating is therefore the visible sign that your body has successfully mobilized its resources and is now trying to stabilize.

Heavy sweating while sick: what steps should you take?

Perspiration is a defense mechanism of the body that should not systematically be blocked.

Except in cases of major discomfort or very high fever, where antipyretic drugs may become necessary, allowing the sweat to flow means allowing the body to complete its healing cycle. By trying to artificially suppress this sweating, we risk disrupting natural temperature regulation and prolonging the state of fatigue. Instead, sweat should be seen as a way for the body to release the excess energy accumulated during the fight against the infection.

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

≈ 1 liter per day

of sweat lost per day on average without physical activity.

≈ 2.5 liters per hour

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

A common mistake when you’re sick is to bundle up too much, either to try to feel better or to “sweat it out.” However, when you wrap yourself in multiple layers of synthetic blankets, you prevent air from circulating and sweat from evaporating. Yet it’s precisely this transition from liquid to gas that actually cools the body. To support sweating without interfering with it, choose loose-fitting clothing made from natural fibers such as cotton. As soon as you feel sweaty, don’t hesitate to take a good shower and change your clothes and bedsheets. Sleeping in a damp environment is not only uncomfortable, it also does not help bring down a fever any faster.

Note : It’s also best to avoid “sweating it out” by doing sports or intense exercise. Forcing physical exertion on a body already mobilized by the immune response divides its energy resources. Instead of speeding up recovery, exercise wears your body out, increases the risk of dehydration, and ultimately makes it much less effective at fighting the infection. The priority is still to rest, so your immune system can function properly.