Why Heat Makes POTS Worse, and How to Build Your Tolerance Back

What Does Heat Do to Brain Blood Flow?

Heat lowers blood velocity in the brain's arteries in people without POTS. In 9 healthy adults, passive heating raised core temperature by 1.1 degrees C and lowered blood velocity in the middle cerebral artery by 8 cm/s while blood pressure held steady (Low, 2008). A second study of 7 healthy adults found the same drop with a rise of about 1.3 degrees C in internal temperature (Brothers, 2009a).

Heat also makes standing harder to tolerate. A review concluded that heat stress "profoundly and unanimously reduces orthostatic tolerance" (Schlader, 2016), and in 13 adults the size of that loss varied widely from person to person (Lee, 2014a).

In our team's view, people with POTS start with a thinner margin. About half of people with POTS run low CO2 when they stand (Stewart, 2023). In the group with low CO2, the lowest brain blood velocity fell to 48 cm/s before CO2 dropped, against 61 cm/s in POTS with normal CO2 and 60 cm/s in healthy controls (Stewart, 2023). A 2024 retrospective study of 125 people with POTS, 127 with hypocapnic cerebral hypoperfusion (orthostatic intolerance without the fast heart rate) and 42 controls concluded in its title that the two conditions represent a spectrum of the same disorder (Novak, 2024).

In our team's view, the heart rate rise in POTS is a compensation that keeps blood moving to the brain. POTS is that compensation succeeding, and orthostatic hypotension is it failing. Heat adds a second demand on the same system. The heat studies above measured healthy volunteers, so how far they carry over to POTS is the clinic's inference from the mechanism.

Does Heat Pull Blood Away From the Brain?

In our team's working model, a system that already struggles to keep enough blood flowing to the heart and brain has less to spare when heat pulls more of that flow into the skin to manage temperature, because it "takes it out of the budget." Heat pulling blood into the skin, our team says, can make some people feel like their battery has been drained.

The measurements support part of that model and complicate the rest. In 11 men, falling CO2 explained the whole drop in brain blood velocity during passive heating, and two-thirds of it when the men were also under orthostatic stress at the point of heat intolerance (Lucas, 2018). Another study found that lower CO2 contributed to the drop, and that other factors, perhaps raised sympathetic nerve activity, were also involved (Brothers, 2009a). In 12 young, healthy adults sitting upright for 50 minutes in a 40 degrees C room (core temperature up 0.2 degrees C), skin blood flow rose and flow to the retina fell, while flow in the internal carotid artery, which supplies most of the brain, didn't change (Caddy, 2025). So the skin takes more blood in mild heat, and those measurements didn't show it coming out of the brain's main supply. Our team holds the budget idea as a working hypothesis and treats CO2 as the measured lead driver.

Rest Heated Heated + cooled

Replay

Research on how POTS skin vessels respond to heat is mixed. In low-flow POTS, the nitric oxide-dependent part of the skin's response to local heating was markedly reduced (Stewart, 2007; Medow, 2005). In a normal-flow POTS group, the same plateau was increased (Stewart, 2011). The 2 groups disagree, so no single statement about skin blood flow fits everyone with POTS.

Two other things heat didn't do in these studies. The brain's response to a CO2 rise held up in heat in 2 studies (Lee, 2014b; Low, 2008), while the response to a CO2 fall was smaller in heat in a third (Lee, 2014a). In a fourth, heat didn't impair dynamic autoregulation during large swings in blood pressure (Brothers, 2009b).

We found no published study that explains heat intolerance in POTS. A PubMed search of titles and abstracts for heat intolerance, heat sensitivity, temperature intolerance or heat exposure together with POTS terms returned 24 results. By title, 7 name POTS or autonomic disorders, and none was a mechanism study. Heat intolerance appeared in them as a listed symptom or as the target of a wearable (PubMed searches, 2026). Studies of sweating in POTS exist (in one, 70 of 211 patients had reduced sweat output on a sweat test), but none we found tested heat tolerance (Zhang, 2022).

Why Do I Swing Between Hot and Cold?

Our team's model is a thermostat with a wider band. A home thermostat set to 70 degrees that doesn't switch the cooling on until 74, or the heat until 65, swings further before it corrects. In the clinic's model, the signals from the body to the brain get less clear in some people with dysautonomia, including many with POTS, and the paraventricular nucleus (PVN), which helps distribute the body's energy-producing systems and sympathetic outputs, has a harder time with it. Sleep sensitivity, hormone regulation, glucose and pH get less stable, and so does the ability to hold temperature steady.

This is the clinic's model. We found no published study of it: a PubMed search of titles and abstracts for POTS terms together with paraventricular or hypothalam* and thermoregulat* or temperature returned 8 results, and none was about thermoregulation in POTS (PubMed searches, 2026).

Our team has seen the pattern in clinic. One patient described going from sweaty to reaching for a blanket and back to sweaty in a day, in an office whose temperature hadn't changed, and the team saw it as her system struggling to manage the swings once it was strained. Another patient, years ago, had a big extension injury after a slip and fall. Lying on one side at night she got very cold, and rolled onto the other side she threw off the blankets in a pool of sweat. She could reproduce it by changing sides. For our team, that case was an early clue to how much temperature control depends on the body signaling to the brain.

Why Do I Get Dizzy in the Shower?

A hot shower is a heat load you take standing still. In our team's working model, hot water pulls more blood into the skin, which leaves less for the brain once you're upright. We found no study of showers in POTS: a PubMed search of titles and abstracts for POTS terms together with shower, bathing, hot water or hot bath returned 0 results (PubMed searches, 2026).

The nearest evidence is from healthy people and bathing. An emergency medicine review of hot baths, hot springs and saunas describes peripheral vasodilation and relative hypovolemia that can trigger syncope, hypotension and falls (Yokoyama, 2026). In 9 young men sitting in a 40 degrees C bath up to the shoulders, blood pressure fell during the bath, and the authors proposed that fainting in hot baths follows a drop in sympathetic tone about 4 minutes after getting in (Nagasawa, 2001). Those studies tested baths, not showers, and none involved POTS, so how far they carry over is the clinic's inference. If you get dizzy or nearly faint in the shower, tell your care team, because a fall in the bathroom is the practical risk.

What Does Standard POTS Care Say About Heat?

A 2026 JAMA review includes "avoidance of heat exposure, dehydration, and prolonged standing" among the first-line steps for POTS (Chung, 2026), and the same advice sits in our POTS treatment guide. On a bad day, that advice is reasonable. Our team's approach works on the regulation underneath and then builds sun tolerance, and none of the POTS results in the search above tested building heat tolerance.

Fluids matter after heat. In 8 volunteers who were dehydrated after about 3 hours in 45 degrees C heat, standing produced a bigger and longer drop in brain blood velocity (28.0% below baseline for 9.0 seconds) than when they had replaced their fluids (20.0% for 6.6 seconds) (Carter, 2006). In 11 men under combined heat and orthostatic stress, mild dehydration didn't change brain blood velocity or cardiovascular measures (Lucas, 2018). For amounts, see how much salt and water helps in POTS.

A thermal wearable has a pilot result. In an open-label study of 22 people with POTS, 4 weeks with a thermal comfort wearable (the Embr Wave2) brought sustained improvements across several domains, including work, leisure activities and enjoyment of life, with no significant change in COMPASS-31 autonomic scores and no control arm (Miglis, 2026).

If a medication helps you on hot days, that's worth keeping, and decisions about it belong to your prescriber.

How Does Our Team Build Heat and Sun Tolerance?

The clinic works on the regulation underneath, because it sees heat intolerance as an output of how well the brain manages blood flow and automatic functions. Alongside that, the team favors sunlight, particularly in the morning, before about 10 a.m. In the team's words, that hour brings less UV and more red light, and the heat is lower and easier to tolerate. We found no published study of morning sun in POTS: a PubMed search of titles and abstracts for POTS terms together with sunlight, sun exposure, heliotherapy or sunshine returned 0 results (PubMed searches, 2026).

For most people, our team says, sunlight is the best move. For patients who aren't mobile yet or don't do well in the sun because of blood flow distribution problems, the team uses artificial red light as a bridge (see red light therapy for dysautonomia). The search for POTS terms together with red light, photobiomodulation or light therapy returned 3 results, and none tested it in POTS (PubMed searches, 2026).

Cooling has more physiology behind it. In 9 healthy adults on a 60-degree tilt while heated, mean arterial pressure and brain blood velocity fell and 4 of the 9 developed presyncopal symptoms, while no one did when the skin was cooled (Wilson, 2002). In 9 healthy men heated to 1 and 2 degrees C above baseline (6 reached 2), skin cooling at the 2 degree level raised end-tidal CO2 by about 4.3 mmHg and lowered cerebral vascular resistance, and it helped more under severe heat than moderate heat (Lucas, 2010). In a pilot of cool head-out water immersion, middle cerebral artery velocity rose from 61 cm/s before to 68 cm/s during immersion, and it was still higher afterward (Worley, 2024). Those were healthy adults, with immersion up to the head rather than the chest, and none had POTS.

1. Morning sun Before about 10 a.m., when the heat is lower and easier to take.
2. Cool water to the chest Cool water to the chest, which our team expects to move blood toward the head.
3. Slow, reclined exit A smooth, graded exit that starts reclined, for example in a zero gravity chair.

The clinic's explanation for the water step is a toothpaste tube: chest-deep water squeezes blood upward toward the head, and cooling the skin narrows its blood vessels so more blood stays in the core and head. That is the team's model, and none of the studies above tested it in POTS or with water at chest level. Our team's caution: cold water and bright sun can both stress a system that's already stretched, so check with your care team before trying either.

Heat exposure can also build tolerance in healthy people. In a randomized trial of 20 endurance athletes, a heat acclimation program raised orthostatic tolerance from 28 to 40 minutes, against 30 to 33 minutes in the control group (Parsons, 2024). Those athletes didn't have POTS, so the trial shows what's possible in healthy people. It can't tell us what will happen in POTS.

Heat Intolerance in POTS: FAQ

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