Heatwaves and your body: what heat does to heart rate, sleep and your data

The heat is here, and your body is paying for it

Late June 2026 brought one of the most intense early-summer heatwaves on record to Central Europe. The Czech Republic logged its hottest day on record, 41.9 °C at Doksany north of Prague, beating the previous national mark by 1.5 °C, as a stagnant heat dome settled over the region. When the air outside climbs toward 40 °C, your body has one priority above all others: keep your core temperature near 37 °C. Almost everything you see on your wearable during a heatwave, a higher heart rate, lower HRV, worse sleep, flows from that single task.

Why your heart works harder in the heat

Cooling down is expensive. To shed heat, your body widens the blood vessels near the skin (vasodilation) and pushes more blood to the surface, where it can release warmth and feed the sweat glands. That blood has to come from somewhere, so your heart beats faster and harder to keep supplying your muscles and organs at the same time. The result is a measurable rise in resting heart rate, often several beats per minute above your normal, simply from sitting still in a hot room.

At the same time, your heart rate variability (HRV) falls. Heat shifts your autonomic nervous system toward the sympathetic ("fight or flight") side and pulls back parasympathetic ("rest and digest") activity. Because HRV reflects that vagal, parasympathetic tone, it drops under thermal strain. In a controlled study, adults exposed to 40 °C for a full day showed reduced HRV indices such as RMSSD and high-frequency power, with older adults affected more than younger ones (Carrillo et al., 2024). In short: to your nervous system, hot weather looks a lot like stress.

Heat steals your deep sleep

To fall asleep, your core temperature has to drop by about one degree, and your body does that by sending heat out to the skin and the surrounding air. In a hot bedroom there is nowhere for that heat to go, so sleep onset is delayed and sleep becomes lighter and more broken. A 2024 systematic review of ambient heat and sleep found that higher night-time temperatures consistently reduce sleep efficiency, cut slow-wave (deep) sleep and REM sleep, and increase the number of awakenings. Large studies of wearable and survey data point the same way: warmer nights mean shorter, more fragmented sleep.

That matters beyond feeling groggy. Deep sleep is when your body does most of its physical repair, and a run of hot nights quietly adds to your sleep debt.

Is it your data, or your body?

This is the question worth answering carefully, because the honest answer has two parts.

The physiological change is real. Your higher resting heart rate, lower HRV and worse sleep during a heatwave are genuine. Your body really is under more strain. So when your recovery or readiness score dips during a hot spell, that is not a glitch, it is the number doing its job.

The sensor is mostly fine. You might expect heat and sweat to confuse the optical (green-light) sensor on your watch. In theory, heat-driven vasodilation changes blood flow near the skin and could affect the signal. In practice, a 2026 validation study of ten commercial wearables across different climate conditions found no significant difference in resting heart-rate accuracy between hot and cool environments. The biggest sources of optical-sensor error are motion and skin tone, not ambient temperature (Bent et al., 2020). The main caveat is exercise: vigorous movement plus heavy sweat can loosen the watch and add motion artifacts, so a mid-workout reading in the heat is the least reliable moment, not the resting overnight numbers your recovery score relies on.

How to read it in MyBodyAI

During a heatwave you will likely see your resting heart rate climb, your HRV slip below its seven-day average, and your morning energy recharge less overnight. MyBodyAI folds those signals into your Training Readiness, so the app may well point you toward an easier day or rest. That guidance is correct: training hard on top of heat strain stacks two stressors on the same nervous system.

One signal to read with context is skin temperature. Devices like Withings and Oura track overnight skin temperature, which MyBodyAI also uses as one input to its Body Alert signal. In a heatwave, an elevated skin temperature is most likely the weather, not an infection. The app weighs several signals together rather than reacting to one, but it is worth knowing that heat can nudge that number, so read it alongside how you actually feel.

Practical tips for training and sleeping through a heatwave

• Move your training to the cooler hours. Early morning or late evening, and drop the intensity. Let readiness, not your plan, set the ceiling.
• Cool the bedroom before sleep. Ventilate, shade the windows during the day, and use a fan or a cool shower so your core temperature can fall.
• Hydrate steadily, with some electrolytes. Sweat costs you water and salts; replacing only water can leave you low on sodium.
• Give it a week or two. The body partially acclimatizes to heat over one to two weeks (more blood plasma, earlier sweating), so the same workout feels easier later in a hot spell than on day one.
• Know the warning signs. Dizziness, nausea, headache, heavy sweating and weakness are signs of heat exhaustion: stop, cool down and rehydrate. If sweating stops, the skin turns hot and dry, or thinking gets confused, that is heat stroke, a medical emergency, so call for help.

When to skip the outdoor workout

Plain air temperature only tells half the story; humidity and direct sun decide how dangerous the heat really is. The number that matters is the feels-like temperature (the heat index), and for organised sport the Wet Bulb Globe Temperature (WBGT), which both fold humidity in. As a rough guide: once the feels-like temperature passes about 32 °C, shorten and ease any outdoor session and move it to the cooler hours; once it reaches the "danger" band around 39 to 40 °C, skip strenuous outdoor exercise and train indoors or rest. For organised sport a WBGT above roughly 28 °C is high risk, and many events cancel near 32 °C. Two things push the real limit lower: full sun can add up to 8 °C to the feels-like number, and high humidity can drag a 32 °C day straight into the danger zone. If you are unacclimatized, older, unwell, or on medication that affects heat regulation, set your bar lower still.

The bottom line: read the heat, do not fight it

Put the pieces together and the picture is consistent. A heatwave pushes your resting heart rate up, your HRV down and your deep sleep shorter, and those three feed straight into a lower readiness score. None of it is a malfunction: your watch is reading a body that really is working harder to stay cool, and the app is right to steer you toward an easier day. The smart move is not to override it but to work with it, train in the cool hours, protect your sleep, drink enough, and give your body the week or two it needs to adapt. Read your numbers in the context of the weather, and a hot spell becomes information instead of a frustration.

FAQ

Why is my resting heart rate higher during the heatwave?
Your heart pumps faster to move blood to the skin for cooling. A few extra beats per minute at rest is a normal response to heat.

Why did my HRV drop?
Heat shifts your nervous system toward sympathetic activity and away from the parasympathetic (vagal) tone that HRV measures, so HRV falls under thermal strain.

Is my watch wrong in hot weather?
Mostly no. Validation studies find ambient heat has little effect on resting heart-rate accuracy. Motion during sweaty exercise is the bigger source of error, not the heat itself.

Should I skip training in a heatwave?
Not necessarily, but lower the intensity, pick the cooler hours, and follow your readiness. If the app points to rest, the heat is part of why.

References

• Carrillo AE et al. (2024). An exploratory investigation of heart rate and heart rate variability responses to daylong heat exposure in young and older adults. Applied Physiology, Nutrition, and Metabolism. PMID: 39255520.
• A systematic review of ambient heat and sleep in a warming climate (2024). Sleep Medicine Reviews.
• Bent B et al. (2020). Investigating sources of inaccuracy in wearable optical heart rate sensors. npj Digital Medicine, 3, 18.
• Accuracy of Optical Heart Rate Measurements for 10 Commercial Wearables in Different Climate Conditions and Activities (2026). JMIR Formative Research, e85186.
• National Weather Service (NOAA). Heat Index and heat safety. weather.gov/safety/heat-index.
• American College of Sports Medicine (2023). Exertional Heat Illness Expert Consensus Statement; WBGT activity-modification guidelines.