A commonly heard advice when someone is down with a fever is to sweat the fever out. You are asked to switch off the fan, don some blankets, wear warm clothes, and just sweat it out. But how far is it true? Why do we sweat during a fever? What does science say? Let’s find out!

Introduction to fever and sweating

Definition of fever

Fever (also called pyrexia) refers to an elevation in body temperature (Normal: 37℃ or 98.6℉) due to infection, inflammation, or other conditions.

It can be categorised as:¹

• Low-grade fever: 37.3 to 38.0℃ (99.1 to 100.4℉)
• Moderate grade fever: 38.1 to 39.0℃ (100.6 to 102.2℉)
• High-grade fever: 39.1 to 41℃ (102.4 to 105.8℉)
• Hyperthermia: Greater than 41℃ (105.8℉)

Purpose of fever

Fever helps the body in fighting infections by activating the cells of the immune system such as natural killer cells, lymphocytes, macrophages, and neutrophils. These cells destroy infectious agents like bacteria and viruses. Hence, fever acts as a natural defence mechanism of the body. 

Role of sweating

Sweating is commonly seen during a fever, with warm, flushed skin, chills, and an increased heart. It helps in regulating temperature by cooling down the skin as sweat evaporates. 

Body temperature regulation mechanism

Your body’s temperature varies by 0.5 degrees Celsius throughout the day. It regulates this temperature naturally via various mechanisms such as:

Homeostasis 

Humans are homeothermic animals, meaning they can self-regulate their internal body temperature despite fluctuations such as exposure to extremely high or low temperatures, hormonal changes, or diseases. This process is called homeostasis and it is regulated by a part of the brain called the hypothalamus. 

Hypothalamus and thermoregulation

The role of the hypothalamus as the body's “thermostat”

The hypothalamus is known as the thermostat of the human body. It is located in the lower middle part of the brain and receives information via nerve cells called thermoreceptors. Thermoreceptors are peripherally present in the skin, mucous membranes, and skeletal muscles, and centrally in the spinal cord and internal organs such as the gut and liver.

Changes in body temperature activate these thermoreceptors and send signals to the brain where the hypothalamus processes this information.

How does the hypothalamus adjust body temperature during illness?

During illness, the immune cells release fever-inducing substances called Pyrogens. 

These pyrogens trigger the hypothalamus to raise the body's set point.  ‘Setpoint’ refers to the optimum temperature of your body at which it functions most efficiently. It is about 37.1℃ or 98.8℉ in temperature.⁵ Your body runs various temperature mechanisms to bring it back to its set point in the case of an increase or decrease.

When the pyrogens trigger a rise in the set point, your body attempts to increase its core temperature via various processes to match the raised set point. This results in the onset of fever. 

Certain disease-causing organisms called pathogens can not survive at a temperature above the body temperature. Hence, a fever inhibits their growth and eventually leads to the death of the pathogen. After the infection has subsided, your body returns to its normal temperature.

How fever alters temperature regulation

Pyrogens and their role

Pyrogens are chemical substances known to induce fever or pyrexia. These pyrogens induce an enzyme called cyclooxygenase 2 (COX 2), which catalyses the formation of prostaglandins (PG) from arachidonic acid. Prostaglandins stimulate the release of neurotransmitter cAMP which increases body temperature.² Milton and Wendlandt were the first to demonstrate the pyrogenic activity of fever due to prostaglandins, specifically PGE2.¹

Internal pyrogens vs. External pyrogens

Pyrogens can be of two types depending on their origin:³

• Internal or endogenous pyrogens: Cytokines (small proteins that play a part in the body’s immune system) formed inside the body that act on the hypothalamus
• External or exogenous pyrogens: Formed by bacteria or viruses and induce interleukins (proteins made by white blood cells that regulate the body’s immune system)

Hypothalamic set point reset

Pyrogens raise the hypothalamic set point

Exogenous pyrogens induce macrophages (a white blood cell that surrounds and kills microorganisms) to release endogenous pyrogens such as IL-1, IL-6, tumour necrosis factor  (TNF), and interferon (IFN). This begins the action of prostaglandin PGE2 and raises the core body temperature leading to fever.^1,2,3

Steps in raising body temperature to match the new set point

• Detection of the new set point: Since the set point is raised, the body detects it as too low and activates various mechanisms to generate or conserve heat
• Heat conservation mechanisms: Reduced sweating and narrowing blood vessels to conserve heat energy
• Heat production mechanism: Shivering and increased metabolic activities such as respiration and digestion
• External heat sources: The person may feel cold and seek warm clothing or blankets

Rising body temperature

Body response to raised set point

Your body responds to the raised set point in the following ways:⁴

• Cold and clammy skin: The lumen of the blood vessels in the limbs is narrowed (vasoconstriction) to reduce heat loss from the body's periphery. This results in cold and pale hands and feet
• Reduces sweating: Initially, less or no sweating is seen to prevent heat dissipation through evaporation
• Shivering: It refers to involuntary muscle contraction and relaxation with increased tone. It usually occurs due to cold temperature, fever, low blood sugar, stress, and anxiety
• Feeling cold even if the external temperature is hot or normal

Phases of fever and sweating regulation

Your body regulates sweat based on the phase of fever. There are 3 phases of fever: 

Onset of fever or initial phase

As we discussed earlier, the initial phase of fever involves reaching a higher set point by conserving and generating heat. It is achieved through reducing sweating, inducing shivering, and vasoconstriction. It may be a sudden or gradual process. 

You may experience chills, headaches, muscle pain, and weakness.⁶

Plateau phase

This is the peak phase, during which the body has reached the new set point. Hence, the core body temperature increases and stabilises at a higher level. 

Heat production is reduced. Heat dissipation is also balanced, and there is minimal to no sweating.⁶

The blood vessels dilate (vasodilation), leading to more blood supply to the limbs accompanied by warm and flushed skin. 

Defervescence (Cooling phase)

After a few hours, when the body has maintained sufficient heat, the hypothalamic set point is lowered, and the fever subsides. This is called the cooling phase. 

Heat dissipation is increased. There is intense sweating and your body returns to normal temperature.⁶ 

Factors influencing sweating during fever

Certain factors influence sweating during fever. They are: 

Severity of infection

The intensity of sweating is based on the severity of the infection. Severe infections over-activate the immune system, leading to excessive sweating to regulate body temperature.

In the case of sepsis, a life-threatening response of the body to severe infection, profuse sweating may be seen.⁷ Similarly, fever with intense sweating can be seen in people affected by meningitis, tuberculosis, and malaria. 

Dehydration

Sweat is composed of mostly water and salt.⁸ It is important to be well-hydrated during illness for effective sweating. Dehydration can aggravate the condition and prevent recovery. 

Individual variability

Age

Sweat production varies with age. Sweat glands deteriorate with increasing age, and this leads to a lesser production of sweat during fever.⁹

Health status

People affected by conditions such as anhidrosis, hypohidrosis, and hypothyroidism produce no to minimal sweat. Similarly, people affected by conditions such as hyperhidrosis, hyperthyroidism, obesity, and menopause may produce more sweat.

Genetics affecting sweat production 

Genetics determine the number and density of sweat glands. People with more sweat glands tend to sweat more, and vice versa. 

Studies show East Asians have fewer sweat glands due to the lack of the ABCC11 gene. This leads to less sweat and odour production.¹⁰

Genetic conditions such as ectodermal dysplasia and Fabry disease are associated with less sweating. 

Medication effects

Antipyretics (fever-reducing drugs) like acetaminophen, ibuprofen, and aspirin lower fever and promote sweating.

Medical perspective on sweating and fever management

Monitor body temperature

If you experience a rise in your body temperature accompanied by chills and weakness, monitor your body temperature several times a day. This helps in avoiding hyperthermia or excessive sweating.

Make sure to use a reliable thermometer for accuracy. 

Use of antipyretics

Antipyretics are the drugs that lower the hypothalamic set point, reduce fever and initiate sweating. They provide relief within a few hours. 

Hydration and electrolyte balance

Adequate water intake plays a key role in hydration and electrolyte balance. For electrolyte imbalance, you can prepare a solution of sugar, salt, and water at home, or buy Oral Rehydration Solution (ORS) sachets from a drugstore. This supports the body’s natural cooling processes.

Summary

A fever is your body’s response to fight an infection. It occurs due to your body’s attempt to reach the new set point raised by the fever-causing substances called Pyrogens. This increases the core body temperature. Your body reduces heat loss via limbs, induces shivering and reduces sweating during its fight with the infection. After the infection has resolved, there is increased heat loss via limbs and excessive sweating to bring the core temperature back to normal. This shows that you cannot sweat out a fever. Sweating, in fact, is a sign of recovery that indicates that the fever is breaking and your body is coming back to normal temperature regulation. Plenty of fluid intake and adequate rest can help you recover soon.