Why do I feel cold and shiver when a fever is starting?

When pyrogens raise the hypothalamic set-point, your body is suddenly below its new target temperature. It responds by narrowing skin blood vessels and shivering to generate and conserve heat - which feels like chills and rigors even as your temperature climbs (Merck Manual; StatPearls).

Why do I sweat, sometimes soaking the sheets, when a fever breaks?

When the set-point resets downward, your body is now above its target, so it dumps the excess heat through sweating and vasodilation - the same way it cools you on a hot day. That heat-shedding is the flush and sweat of a fever 'breaking' (Merck Manual).

Are night sweats always a sign of infection?

No. Night sweats are non-specific. They can accompany any fever-causing condition and are classically linked to tuberculosis and lymphoma, but also occur with many other conditions and with entirely benign causes like a warm room or hormonal changes. They flag that something is driving temperature regulation, not what (Clinical Methods).

Is 98.6F really the normal body temperature?

It is a historical average, not a fixed value. A 1992 study of 148 healthy adults found a mean of 36.8C (98.2F), an upper normal limit of 37.7C (99.9F), and a normal daily swing of about 0.5C - lowest near 6 AM and highest in late afternoon (Mackowiak 1992).

When should a fever or night sweats be checked by a clinician?

A fever that persists or has no clear cause (the pattern investigated as 'fever of unknown origin,' above 38.3C and persisting), or night sweats that are frequent, drenching, or paired with weight loss or lasting fever, should be evaluated. This page is educational and not a substitute for medical care.

Research

Fever and Sweating: The Physiology

Fever is not simply the body overheating - it is the body deliberately raising its own temperature target. Pyrogens from an infection prompt the hypothalamus to reset its thermostat to a higher set-point. To reach it, the body constricts blood vessels and shivers, which is why the start of a fever feels cold and brings chills. When the set-point resets back down - as the illness eases or with antipyretics - the body dumps the excess heat through sweating and flushing. That single mechanism explains the shiver-then-sweat pattern of fevers and the sweat-soaked sheets of feverish night sweats. This page traces the physiology to primary sources.

By the Sweat Explained Editorial Team · Published 2026-07-13 · Last reviewed 2026-07-13 · Educational information, not medical advice.

Key statistics at a glance

~37C / 98.6F
traditional normal core temperature the hypothalamus regulates around
StatPearls, Physiology of Fever
36.8C / 98.2F
actual mean normal oral temperature in a landmark reappraisal; upper limit 37.7C (99.9F)
Mackowiak 1992, JAMA
>=38.0C / 100.4F
common threshold for clinically significant fever (low-grade defined as 37.3-38.0C)
StatPearls, Physiology of Fever
chills up, sweats down
vasoconstriction and shivering raise temperature to the new set-point; sweating and vasodilation shed heat when it resets down
Merck Manual; StatPearls

Fever is a moved thermostat, not just overheating

Fever is defined as the elevation of core body temperature above a set-point regulated by the body's thermoregulatory center in the hypothalamus. The key word is set-point. In a fever the hypothalamus deliberately raises its temperature target, and the body then works to meet that higher target using the same machinery it uses to stay warm on a cold day.

This distinguishes fever from hyperthermia. In fever, the hypothalamus raises the set-point and the body maintains a controlled increase in core temperature toward it. In hyperthermia, core temperature rises beyond the set-point despite the hypothalamus trying to hold the line - the thermostat is being overwhelmed, not reset. The two look similar on a thermometer but are physiologically opposite, and that difference matters for how each is treated.

How the set-point gets raised

The trigger is a class of signaling molecules called pyrogens. Exogenous pyrogens - such as bacteria and viruses - stimulate the body to release endogenous pyrogens including interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor (TNF), and interferons. These act on the hypothalamus by driving the synthesis of prostaglandin E2 (PGE2), which is the molecule that actually shifts the set-point upward.

Because PGE2 is the final common step, blocking its production is how many antipyretic medicines lower a fever: they interrupt the prostaglandin pathway, the set-point falls back toward normal, and the body then sheds its now-excess heat. This is also why a fever tends to return as a dose wears off - if the underlying pyrogens are still present, the set-point climbs again.

Temperature thresholds and fever grades

These bands are the categories used in the StatPearls physiology reference. Individual definitions of fever vary between guidelines, and normal temperature itself varies by person, time of day, and measurement site (see the note below).

Core temperature bands used to grade fever
Category	Temperature range
Normal (traditional)	~36.8-37.0C (98.2-98.6F)
Low-grade fever	37.3-38.0C (99.1-100.4F)
Moderate-grade fever	38.1-39.0C (100.6-102.2F)
High-grade fever	39.1-41.0C (102.4-105.8F)
Hyperthermia / hyperpyrexia	>41C (>105.8F)

The familiar 98.6F (37C) figure is a historical average. A 1992 reappraisal of 148 healthy adults found a mean of 36.8C (98.2F) with an upper limit of normal of 37.7C (99.9F), and a normal daily swing of about 0.5C - lowest around 6 AM, highest in late afternoon (Mackowiak 1992).

Why you shiver on the way up and sweat on the way down

Once the set-point jumps to a higher target, the body is suddenly 'too cold' relative to where the hypothalamus now wants to be. To close the gap it triggers vasoconstriction - shunting blood away from the skin to cut heat loss - and often shivering, which increases heat production. Subjectively this is the chill or rigor at the start of a fever: you pile on blankets and still feel cold even as your temperature is climbing.

When the set-point resets downward - because the pyrogens are clearing or an antipyretic has acted - the body is now 'too hot' relative to the new lower target. It offloads the surplus heat exactly as it would on a hot day: through sweating and vasodilation. This is the flush and the drenching sweat as a fever 'breaks.' The same downward reset happening during the night, on top of the body's normal circadian temperature rhythm, is a core reason fevers so often produce night sweats.

So chills and sweats are not two different problems - they are the two directions of one moving thermostat. Chills accompany a rising set-point; sweats accompany a falling one.

Fever, night sweats, and what they do and do not tell you

Night sweats can be described as nocturnal sweating that results from an exaggeration of the normal circadian temperature rhythm, and they may occur with essentially any condition that causes fever. That makes them a sensitive but non-specific sign: their presence flags that something is driving the thermostat, but not what.

The classic teaching links drenching night sweats to tuberculosis and lymphoma, but the same reference lists many other causes - among them brucellosis, lung abscess, bacterial endocarditis, diabetic autonomic neuropathy, nocturnal hypoglycemia, nocturnal angina, and diabetes insipidus. Many common, benign situations (a warm bedroom, ordinary viral illness, or hormonal changes such as menopause) also produce night sweats. Because the symptom overlaps so many conditions, it is interpreted in context rather than on its own.

When to seek care

Most short-lived fevers accompany self-limiting infections. But some patterns warrant medical attention. A persistent or unexplained fever - for example, one lasting beyond a couple of weeks without an obvious cause, the situation clinicians investigate as 'fever of unknown origin' (a temperature above 38.3C that persists) - should be evaluated. So should drenching, recurrent night sweats, especially when paired with unexplained weight loss, persistent fever, or new lumps.

This page explains normal physiology; it is not a diagnosis or a substitute for medical care. Anyone with a high fever, a fever that will not resolve, or night sweats that are frequent, soaking, or unexplained should speak with a clinician, who can weigh the full clinical picture.

Methodology and limitations

This page compiles standard physiology references: the StatPearls 'Physiology, Fever' chapter and the NCBI Clinical Methods chapter 'Fever, Chills, and Night Sweats,' plus the Merck Manual Professional article on fever for the vasoconstriction/sweating mechanism, and Mackowiak et al. (JAMA 1992) for the reappraisal of normal body temperature. Each figure and mechanism statement was traced to its source and confirmed.

Limitations: temperature thresholds for grading fever vary between references and clinical settings, and 'normal' body temperature varies with the individual, time of day, and how and where it is measured - so the bands shown are conventions, not hard biological boundaries. Night sweats are non-specific and cannot identify a cause on their own. Nothing here is a diagnosis or medical advice; it describes physiology for general education.

Frequently asked questions

Why do I feel cold and shiver when a fever is starting?: When pyrogens raise the hypothalamic set-point, your body is suddenly below its new target temperature. It responds by narrowing skin blood vessels and shivering to generate and conserve heat - which feels like chills and rigors even as your temperature climbs (Merck Manual; StatPearls).
Why do I sweat, sometimes soaking the sheets, when a fever breaks?: When the set-point resets downward, your body is now above its target, so it dumps the excess heat through sweating and vasodilation - the same way it cools you on a hot day. That heat-shedding is the flush and sweat of a fever 'breaking' (Merck Manual).
Is fever the same thing as overheating (hyperthermia)?: No. In fever the hypothalamus raises the set-point and the body climbs toward it in a controlled way. In hyperthermia, core temperature rises beyond the set-point despite the hypothalamus trying to hold it - the regulation is overwhelmed rather than reset (StatPearls).
Are night sweats always a sign of infection?: No. Night sweats are non-specific. They can accompany any fever-causing condition and are classically linked to tuberculosis and lymphoma, but also occur with many other conditions and with entirely benign causes like a warm room or hormonal changes. They flag that something is driving temperature regulation, not what (Clinical Methods).
Is 98.6F really the normal body temperature?: It is a historical average, not a fixed value. A 1992 study of 148 healthy adults found a mean of 36.8C (98.2F), an upper normal limit of 37.7C (99.9F), and a normal daily swing of about 0.5C - lowest near 6 AM and highest in late afternoon (Mackowiak 1992).
When should a fever or night sweats be checked by a clinician?: A fever that persists or has no clear cause (the pattern investigated as 'fever of unknown origin,' above 38.3C and persisting), or night sweats that are frequent, drenching, or paired with weight loss or lasting fever, should be evaluated. This page is educational and not a substitute for medical care.

Sources

Primary peer-reviewed studies and official sources first, then reviews and institutional framing (secondary).

Balli S, Shumway KR, Sharan S. Physiology, Fever. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; NBK562334. StatPearls
Yagupsky P (author of chapter series); Walker HK, Hall WD, Hurst JW (eds). Fever, Chills, and Night Sweats. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Chapter 211; NBK324. NCBI Bookshelf
Merck Manual Professional Version. Fever (Biology of Infectious Disease). Vasoconstriction/shivering on set-point rise; sweating/vasodilation on downward reset. Merck Manual
Mackowiak PA, Wasserman SS, Levine MM. A critical appraisal of 98.6 degrees F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich. JAMA. 1992;268(12):1578-1580. PMID: 1302471. PubMed

How to cite this page

Sweat Explained. Fever and Sweating: The Physiology. Published 2026-07-13; last reviewed 2026-07-13. Available at: https://sweatexplained.com/research/fever-and-sweating-physiology

Please cite the original studies for the underlying figures. Journalists are welcome to link to this page; the charts are original renderings of the cited data.