Altitude lowers heart rate variability (HRV) at first, and the body adapts over weeks. A 2026 meta-analysis pooled 15 studies in which 698 healthy adults went above 2,500 m for up to seven days: SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , 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 , pNN50 pNN50 The percentage of neighbouring NN intervals that differ by more than 50 milliseconds. It tracks fast, vagally driven changes, like RMSSD, but as a share rather than a size. More about pNN50 , high-frequency and low-frequency power LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power all fell compared with sea level, and the LF/HF ratio LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio rose (Li and colleagues, 2026). Flights and jet lag have not been measured the same way; their effect on your number mostly comes through lost sleep and a body clock Circadian rhythm The body's 24-hour clock. Heart rate is lower and vagal HRV higher at night than by day, so readings are only comparable when they are taken at the same time of day. More about Circadian rhythm out of step with the local day.
Why thin air lowers HRV
Higher up, each breath carries less oxygen. The body answers by breathing faster, pumping harder and tilting control toward the 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 (“fight or flight”) branch, so heart rate rises and the vagal brake eases. The review’s authors summed up the pattern as reduced variability, vagal withdrawal and relative sympathetic predominance. A 2024 review of the factors that shape HRV lists altitude among the external conditions linked with lower values (Sammito and colleagues, 2024).
What the studies found
Acute exposure. In the pooled studies, every standard measure fell in the first week above 2,500 m. Above 3,500 m the drop in SDNN and the rise in LF/HF were larger than at lower heights, while RMSSD, pNN50 and HF 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 fell by similar amounts at both. Trained people showed the same vagal drop as untrained ones but kept more low-frequency power, which the authors read as a stronger sympathetic regulatory capacity. Leaving out any one study did not change the result, and the authors found no clear sign of publication bias, though with fewer than 20 studies per measure they could not rule it out.
A modest altitude, over months. Twenty-two medical staff moved from Chengdu (about 500 m) to Yecheng at 1,400 m. After one week, SDNN, SDANN SDANN The standard deviation of the mean NN interval of each 5-minute segment of a 24-hour recording, in milliseconds. It measures the slow changes over a day and ignores everything faster. More about SDANN and SD2 SD1 and SD2 Two measures of the spread of dots in a Poincaré plot, in milliseconds. SD1 is the width and tracks short-term variability, like RMSSD; SD2 is the length and tracks overall variability. More about SD1 and SD2 were lower than before leaving. By three months they had risen, and they rose further in the week after returning home. Average heart rate fell steadily through the stay. Stable sleep time dropped during acclimatisation and only partly recovered after the return (Ye and colleagues, 2024). Even a height many people would not call “altitude” showed a measurable dip and a measurable recovery.
Flights and jet lag. Aircraft cabins are pressurised to the equivalent of roughly 1,800 to 2,400 m, so a long-haul flight is a few hours of mild altitude, with dry air, sitting still and often alcohol on top. Crossing time zones then puts your body clock out of step with local night, which disturbs sleep for days. Sleep loss lowers RMSSD across 11 trials (Zhang and colleagues, 2025), and HRV itself follows the body clock, rising at night and falling in the morning. No published study has followed wearable HRV through jet lag, so the size and length of the effect are unknown.
How to read your own data
- Expect the first week to look low. At altitude, a drop in the first days is the normal response. Compare with your sea-level baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline for interest, but judge recovery by the trend after day three or four.
- Treat a trip as its own short baseline. After a long flight, give yourself two or three nights before reading much into the number. The lesson on building your HRV baseline explains why single nights mislead.
- Watch heart rate and breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate . Both tend to rise at altitude. A falling overnight heart rate over the stay is a sign of acclimatisation.
- Keep it comparable. Measure the same way at the same point in the day. Local time, not home time, is the clock that matters once you have adjusted.
Heat and dehydration often come with the same trips; see does heat lower HRV? For shift-work style disruption of the body clock, read do night shifts lower HRV?
When to talk to a doctor
Altitude sickness is a medical problem, not a tracking one. Headache, nausea, dizziness, breathlessness at rest or confusion above about 2,500 m need rest, descent and medical advice, whatever your HRV shows. Ask a doctor before high-altitude travel if you have a heart or lung condition. HRV cannot diagnose anything, and this page is not medical advice.