Yes, while your whole body is hot. A 2023 review gathered 27 studies that warmed healthy adults and watched their heart’s control: whole-body heating lowered cardiac vagal modulation in 14 of them, and two studies reported a sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system rise alongside the vagal drop (Ferreira and colleagues, 2023). Saunas and local heat behaved differently, and over eight weeks a post-exercise sauna did nothing extra for resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate variability (HRV) (Lee and colleagues, 2025).
Why heat lowers HRV
When you are hot, the body sends blood to the skin to shed heat. The heart has to pump more to fill those extra vessels, so heart rate rises and the sympathetic branch takes a bigger share of control. The vagal brake that creates most beat-to-beat variation eases off, and short-term measures such as RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD and high-frequency power HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power fall. A 2024 review of the factors that shape HRV lists heat among the external factors linked with lower values (Sammito and colleagues, 2024).
Sweat adds a second hit. Lose enough fluid and blood volume falls, which pushes heart rate up further. In 12 trained men, exercising dehydrated lowered high-frequency HRV compared with the same session well hydrated (Hernandez and colleagues, 2026). See does dehydration lower HRV?
Whole-body heat, local heat and saunas
The review separated three kinds of heating:
| Kind of heat | What most studies found |
|---|---|
| Whole-body heating (warming the whole body to raise core temperature) | Lower vagal HRV; some also found more sympathetic activity and a weaker blood-pressure reflex |
| Local heating (part of the body) | Vagal measures tended to rise |
| Sauna bathing | Vagal measures tended to rise |
So “heat” is not one thing. Being hot all over, as on a hot night or during a long session in the sun, pulls HRV down. A sauna session, in the studies the review gathered, tended to leave the vagal side higher.
That short rise does not seem to add up to a lasting one. In a Finnish trial, 38 inactive adults aged around 49 with at least one heart risk factor were assigned to exercise plus a 15-minute sauna, exercise alone, or no change, for eight weeks. Exercise improved two HRV measures compared with no change. Adding the sauna made no further difference to any measure (Lee and colleagues, 2025). More on this in do saunas or cold plunges raise HRV?
Hot nights and summer readings
Your wearable measures HRV while you sleep, and a hot bedroom keeps your core temperature and heart rate up through the night. Expect lower nightly readings during heatwaves and on holiday somewhere hot, and expect them to return when the nights cool. The practical levers are a cooler room, a fan, a lighter cover, and enough fluid during the day. A cool shower before bed helps the body dump heat.
How to read your own data
- Tag hot days and hot nights. After a few weeks you will see your own summer penalty. The lesson on building your HRV baseline shows how to compare.
- Look at heart rate too. A higher overnight heart rate alongside lower HRV fits heat and fluid loss. See HRV vs resting heart rate.
- Keep measurement conditions fixed. A morning reading taken in a sunlit room one day and an air-conditioned one the next adds noise.
- Do not take your daily reading after a sauna. The short vagal rise would not be comparable with your other mornings.
For the wider list of what moves the number, see what affects HRV and HRV and sleep.
When to talk to a doctor
Heat illness is a medical emergency. Get help for confusion, fainting, a very fast heartbeat, or a high temperature with no sweating. Ask a doctor before regular sauna use if you have a heart condition, low blood pressure or take medicines that affect fluid balance. HRV cannot tell you how your body is coping with heat, and this page is not medical advice.