What actually happens during a hot flash?

A hot flash is a heat-dissipation response: the body sweats and widens skin blood vessels (flushing) as if it were overheating, then core temperature dips slightly. Research shows most hot flashes are preceded by a small rise in core body temperature.

What is the thermoneutral zone?

It is the range of core temperature between the sweating threshold (above) and the shivering threshold (below), within which the body makes only minor adjustments and does not sweat or shiver. In women with hot flashes this zone is greatly narrowed, close to zero, compared with about 0.4 °C in women without hot flashes.

Why does a tiny temperature change trigger a hot flash?

Because the buffer is almost gone. With a near-zero thermoneutral zone, the small rises in core temperature that everyone experiences are enough to cross the lowered sweating threshold and set off the cooling response.

How does estrogen fit in?

Hot flashes coincide with the estrogen decline of menopause, and estrogen replacement raises the sweating threshold and reduces flashes in controlled studies. But estrogen levels do not differ between women with and without symptoms, so estrogen withdrawal is a trigger rather than the entire cause.

What are KNDy neurons?

KNDy neurons are hypothalamic neurons that produce kisspeptin, neurokinin B, and dynorphin. They sit near the brain's temperature-control center, become more active when estrogen falls, and are proposed to help narrow the thermoneutral zone that leads to hot flashes.

Is this page medical advice?

No. It is a summary of verified physiology from published sources and does not cover treatment choices. Only a clinician can advise on managing hot flashes.

Research

What Causes Hot Flashes? The Thermoregulation Story

A hot flash is not random heat; it is a heat-dissipation response, the same set of reactions the body uses to cool down, consisting of sweating and a rush of blood to the skin. Research using precise core-temperature measurement shows that most hot flashes are preceded by a tiny rise in body temperature. The reason that small rise sets off a full cooling response is a narrowed thermoneutral zone: the temperature band between the sweating and shivering thresholds is much smaller in symptomatic women. This page explains that physiology from the peer-reviewed literature. It is educational only and is not a diagnosis or medical advice.

By the Sweat Explained Editorial Team · Published 2026-07-20 · Last reviewed 2026-07-20 · Educational information, not medical advice.

Key statistics at a glance

0 vs 0.4 °C
the thermoneutral (null) zone was near zero in symptomatic postmenopausal women versus about 0.4 °C in those without hot flashes
Freedman & Krell 1999 (Am J Obstet Gynecol)
raised threshold
estrogen replacement raised the core-temperature sweating threshold and reduced hot flash occurrence in a controlled study
Freedman & Blacker 2002 (Fertil Steril)
small Tc rise
most hot flashes are preceded by small elevations in core body temperature acting within the reduced zone
Freedman 2001 (Am J Hum Biol)
noradrenergic
elevated central noradrenergic activation narrows the thermoneutral zone; clonidine widens it and eases hot flashes
Freedman 2005 (Semin Reprod Med)
KNDy neurons
hypothalamic KNDy (kisspeptin, neurokinin B, dynorphin) neurons are proposed to link estrogen loss to the thermoregulatory changes
Rance 2013 (Front Neuroendocrinol)

A hot flash is a cooling response

Physiologically, a hot flash looks like the body trying to lose heat. It combines sweating on the face, neck, and chest with peripheral vasodilation, a widening of skin blood vessels that produces the flushed, warm feeling, followed by a small drop in core temperature as heat is shed. These are the same responses the body mounts when it is genuinely overheated (Freedman, 2001).

The puzzle researchers set out to solve was why this cooling response fires when the person is not actually too hot. Using a rapidly responding ingested telemetry pill to track core temperature, investigators found that most hot flashes are preceded by a small rise in core body temperature. That observation reframed the hot flash as a triggered heat-loss reflex rather than a spontaneous event.

The thermoneutral zone explained

Core temperature is normally regulated between an upper sweating threshold and a lower shivering threshold. The gap between them is the thermoneutral zone (also called the null zone): within it, small temperature changes are handled quietly by adjusting skin blood flow, without sweating or shivering. In most people this buffer gives the body room to fluctuate without triggering a major response.

The key finding is that this zone is dramatically narrowed in women with hot flashes. Freedman and Krell (1999) measured a thermoneutral zone of essentially zero in symptomatic postmenopausal women, compared with about 0.4 °C in asymptomatic women, driven mainly by a lowered sweating threshold. With almost no buffer, the small temperature rises that everyone experiences are enough to cross the sweating threshold and set off a hot flash.

Symptomatic versus asymptomatic women

Findings comparing postmenopausal women with and without hot flashes (Freedman & Krell, 1999; Freedman, 2001).

The thermoneutral zone and hot flash triggering
Feature	Women with hot flashes	Women without hot flashes
Thermoneutral (null) zone	Essentially zero	About 0.4 °C
Sweating threshold	Lowered, easily crossed	Higher, harder to reach
Effect of a small Tc rise	Crosses threshold and triggers a hot flash	Absorbed within the buffer, no flash
Central noradrenergic activity	Elevated	Lower
Response to peripheral cooling	Reducing core temperature helps prevent flashes	Not applicable

Tc means core body temperature. The near-zero zone in symptomatic women is why a rise too small to notice can set off a full cooling response.

What narrows the zone

The narrowing is not explained by estrogen alone. Research points to several interacting contributors:

Estrogen withdrawal. Hot flashes coincide with the estrogen decline of menopause, and estrogen replacement raises the sweating threshold and reduces flashes. But estrogen levels do not differ between symptomatic and asymptomatic women, so withdrawal is a trigger, not the whole story (Freedman & Blacker, 2002).
Central noradrenergic activation. Elevated brain noradrenaline narrows the thermoneutral zone in animal studies. Clonidine, which lowers noradrenergic activity, widens the zone and eases hot flashes, supporting this mechanism (Freedman, 2005).
Small core-temperature fluctuations. Normal rises in core temperature, from activity, warmth, or circadian variation, provide the trigger that crosses the lowered threshold.
Hypothalamic KNDy neurons. Neurons producing kisspeptin, neurokinin B, and dynorphin sit near the brain's temperature-control center and are proposed to link estrogen loss to the narrowed zone (Rance et al., 2013).

The hypothalamic KNDy hypothesis

More recent work has focused on a group of neurons in the hypothalamus, close to the brain's thermoregulatory center, that produce kisspeptin, neurokinin B, and dynorphin, together nicknamed KNDy neurons. When estrogen falls at menopause, these neurons enlarge and become more active. Because they signal to the temperature-regulating region, their overactivity has been proposed as a mechanism that narrows the thermoneutral zone and links estrogen loss to hot flashes (Rance et al., 2013).

This hypothesis has been strengthened by research on neurokinin-3 receptor signaling, the pathway KNDy neurons use, which has become an active area of study in the physiology of vasomotor symptoms. The mechanistic detail matters here for one reason: it shows the hot flash is a specific, traceable thermoregulatory event rather than a vague symptom. Treatment options are covered on separate pages linked below.

What this means in plain terms

A hot flash is the body running its cooling program at the wrong time. In women with hot flashes, the temperature buffer between sweating and shivering is almost gone, so a small, ordinary rise in core temperature is enough to trigger sweating and flushing. Estrogen withdrawal, elevated brain noradrenaline, and hypothalamic KNDy neuron activity all appear to contribute to that narrowed buffer.

Understanding the physiology also explains why cooling the body and reducing triggers of core-temperature rise are consistent themes in the research. This page describes mechanism only; it summarizes verified figures from published sources and is educational, not a diagnosis or medical advice.

Methodology and limitations

This page draws on Freedman & Krell 1999 (Am J Obstet Gynecol 181:66-70, the null-zone measurement), Freedman & Blacker 2002 (Fertil Steril 77:487-490, estrogen and the sweating threshold), Freedman 2001 (Am J Hum Biol 13:453-464, physiology of hot flashes), Freedman 2005 (Semin Reprod Med, pathophysiology review), Freedman 2014 (J Steroid Biochem Mol Biol 142:115-120, mechanisms review), and Rance et al. 2013 (Front Neuroendocrinol 34:211-227, the KNDy hypothesis). Each figure (0 versus 0.4 °C thermoneutral zone, raised sweating threshold with estrogen, small Tc rises as triggers, noradrenergic narrowing, and KNDy neurons) was traced to those sources.

Limitations: the thermoneutral-zone measurements come from small, carefully controlled laboratory studies (roughly 12 symptomatic and 8 asymptomatic women in the key experiment), so exact values should be read as representative rather than universal. The KNDy hypothesis is a well-supported mechanism but remains an area of active research. This page describes physiology, not treatment, and it is not a diagnosis or medical advice.

Frequently asked questions

What actually happens during a hot flash?: A hot flash is a heat-dissipation response: the body sweats and widens skin blood vessels (flushing) as if it were overheating, then core temperature dips slightly. Research shows most hot flashes are preceded by a small rise in core body temperature.
What is the thermoneutral zone?: It is the range of core temperature between the sweating threshold (above) and the shivering threshold (below), within which the body makes only minor adjustments and does not sweat or shiver. In women with hot flashes this zone is greatly narrowed, close to zero, compared with about 0.4 °C in women without hot flashes.
Why does a tiny temperature change trigger a hot flash?: Because the buffer is almost gone. With a near-zero thermoneutral zone, the small rises in core temperature that everyone experiences are enough to cross the lowered sweating threshold and set off the cooling response.
How does estrogen fit in?: Hot flashes coincide with the estrogen decline of menopause, and estrogen replacement raises the sweating threshold and reduces flashes in controlled studies. But estrogen levels do not differ between women with and without symptoms, so estrogen withdrawal is a trigger rather than the entire cause.
What are KNDy neurons?: KNDy neurons are hypothalamic neurons that produce kisspeptin, neurokinin B, and dynorphin. They sit near the brain's temperature-control center, become more active when estrogen falls, and are proposed to help narrow the thermoneutral zone that leads to hot flashes.
Is this page medical advice?: No. It is a summary of verified physiology from published sources and does not cover treatment choices. Only a clinician can advise on managing hot flashes.

Sources

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

Freedman RR, Krell W. Reduced thermoregulatory null zone in postmenopausal women with hot flashes. Am J Obstet Gynecol. 1999;181(1):66-70. Thermoneutral zone near zero in symptomatic women versus about 0.4 °C in asymptomatic women. PubMed
Freedman RR, Blacker CM. Estrogen raises the sweating threshold in postmenopausal women with hot flashes. Fertil Steril. 2002;77(3):487-490. Estrogen replacement raised the core-temperature sweating threshold and reduced hot flash occurrence. Journal
Freedman RR. Physiology of hot flashes. Am J Hum Biol. 2001;13(4):453-464. Hot flashes are a heat-dissipation response triggered by small core-temperature rises within a reduced thermoneutral zone. Journal
Freedman RR. Pathophysiology and treatment of menopausal hot flashes. Semin Reprod Med. 2005;23(2):117-125. Reviews the reduced thermoneutral zone and elevated central noradrenergic activation. (secondary) Journal
Freedman RR. Menopausal hot flashes: mechanisms, endocrinology, treatment. J Steroid Biochem Mol Biol. 2014;142:115-120. Synthesis of the thermoregulatory and noradrenergic mechanisms of hot flashes. (secondary) Full text
Rance NE, Dacks PA, Mittelman-Smith MA, Romanovsky AA, Krajewski-Hall SJ. Modulation of body temperature and LH secretion by hypothalamic KNDy neurons: a novel hypothesis on the mechanism of hot flushes. Front Neuroendocrinol. 2013;34(3):211-227. Proposes that hypothalamic KNDy neurons link estrogen withdrawal to thermoregulatory changes. (secondary) Full text

How to cite this page

Sweat Explained. What Causes Hot Flashes? The Thermoregulation Story. Published 2026-07-20; last reviewed 2026-07-20. Available at: https://sweatexplained.com/research/hot-flash-physiology-thermoneutral-zone

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.