Quick Answer: In heat, a large share of your blood volume is redirected to the skin to shed heat, and warm-weather sweating reduces plasma volume. Less blood returning to the heart means less pumped per beat, so heart rate rises to maintain the same output. It is a circulatory problem, not a fitness one. Expect several beats per minute of elevation in warm conditions and pace by effort instead.

Your heart is not working harder because you got slower.

It is working harder because it has less to work with.

What is actually happening in the circulation?

Two things at once, and they compound.

Blood gets diverted to the skin. Cooling requires warm blood at the surface, so a substantial fraction of your cardiac output goes to skin circulation rather than muscle. That blood is doing thermal work, not mechanical work.

Plasma volume falls with sweating. Sweat is drawn from the fluid portion of your blood, so sustained sweating reduces total blood volume. Less volume means less filling of the heart between beats.

Stroke volume drops as a result. With less blood returning, each beat ejects less. To keep cardiac output constant, rate has to rise - which is exactly what your watch is showing you.

This is cardiovascular drift. Within a single session it appears as a heart rate that climbs steadily at unchanged pace. Across a hot season it appears as a higher baseline for every run.

And it stacks with anything that lowers fluid. Starting slightly dehydrated, alcohol the night before, or a poor night’s sleep all push it further. The same run on a bad day can sit well above your normal.

Humidity makes it substantially worse. When the air is already saturated, sweat cannot evaporate, so the cooling never happens and you keep sweating for no thermal benefit. A humid warm day is far harder than a dry hot one at the same temperature.

How much elevation is normal?

Enough to change your training zones entirely, which is why people misread it.

A modest rise in core temperature produces a clear rise in heart rate. In practice, warm conditions commonly add several to ten-plus beats per minute at an identical easy pace.

Which pushes easy runs into a harder zone. Your genuinely easy pace now reads as moderate on your watch. Following the watch means running slower; following the pace means training harder than intended.

The drift within a session is separate and additive. Even in stable conditions, heart rate typically climbs across a long steady effort as plasma volume falls. An hour into a warm run, both effects are present.

It is worse if you are less heat-acclimatised. People who train mostly indoors or early in the morning show a bigger effect when they run in the afternoon. Acclimatisation is real and takes a week or two of repeated exposure.

And it reverses quickly. The first genuinely cool week of autumn typically returns heart rate to normal within days. People routinely report a personal best in the first cold snap - that is the plasma volume coming back, not a sudden fitness jump.

What should you actually do about it?

Change the metric, the timing, and the hydration - in that order.

Pace by effort, not by heart rate, in the heat. Conversational should mean conversational. If you can speak in full sentences, the run is easy, whatever the number says.

Or use pace with an explicit heat allowance. Adding a meaningful margin per mile or kilometre in warm conditions keeps the physiological load where you intended it. The allowance grows with humidity, not just temperature.

Move hard sessions to early morning. Not just for temperature - overnight cooling also lowers humidity, and the combination matters more than either alone. Early morning is usually the best training window of a warm day by a clear margin.

Drink to a plan on longer efforts. Sipping regularly through a long warm session limits the plasma volume loss and blunts the drift meaningfully. Waiting for thirst means you are already behind.

Include sodium if you sweat heavily or for a long time. Plain water in large volumes over a long hot effort dilutes sodium, and the result is worse rather than better. An electrolyte drink for anything long and hot.

And accept a lighter block in peak heat. Trying to hold winter paces through the hottest weeks tends to produce accumulated fatigue rather than fitness. Maintain frequency, reduce intensity, and let autumn be the block where you push.

What Nobody Tells You About Heat and Heart Rate

Your fitness is fine and the data proves it, if you look at the right number. Pace at a given effort, in comparable conditions, is the honest measure. Heart rate at a given pace is contaminated by the weather.

Heat acclimatisation is a genuine training adaptation. After a week or two of consistent heat exposure, plasma volume expands, you sweat earlier and more efficiently, and heart rate at the same effort comes down measurably. The first hot week is always the worst week.

Which means the adaptation carries into cool weather. The expanded plasma volume does not vanish immediately, and this is why athletes use heat exposure deliberately. You are getting something out of a hard summer, even if it does not feel like it.

The morning-versus-evening difference is larger than people expect. The same run at 6am and 6pm on the same day can differ substantially in heart rate. If you compare across times of day, you are comparing two different sessions.

Sleep affects it more than you would guess. A poor night’s sleep raises heart rate at a given effort noticeably, and warm nights produce poor sleep. In a heatwave, both effects arrive together.

And the reversal is dramatic enough to be motivating. Hold your training through the warm weeks and the first cool morning will feel like someone removed a weight. Autumn is where a hot summer pays out. If your heart rate is very high at easy effort even in cool conditions, or you get chest discomfort or unusual breathlessness, that is worth getting checked. This is general information, not medical advice.

FAQ: Heat and Heart Rate, Answered

How long does heat acclimatisation take?

Most of the adaptation happens within one to two weeks of regular exposure, with the plasma volume expansion arriving earliest. It also fades within a couple of weeks once the exposure stops, which is why a week indoors mid-summer sets you back.

Should I train by heart rate at all in summer?

Use it as a ceiling rather than a target. Cap your easy runs at a heart rate you will not exceed and let pace fall wherever it falls. That preserves the easy-hard distinction, which is the thing that actually matters.

Why is humidity worse than temperature?

Because evaporation is how sweating cools you, and saturated air prevents it. You lose the fluid without getting the cooling - the worst possible combination, and the reason a humid warm day beats a dry hot one for difficulty.

TL;DR:

• Blood diverted to skin plus reduced plasma volume means less pumped per beat, so rate rises.
• Expect several to ten-plus beats per minute of elevation at identical easy pace in warm weather.
• Pace by effort, not heart rate - conversational is the test that still works.
• Humidity is worse than heat because sweat cannot evaporate and cool you.
• It reverses within days of the first cool week, and the heat adaptation carries over.

For the rest of this warm spell, log the temperature next to every run. The pattern will make the numbers obvious.