Sleeping at altitude: why your data looks alarming and mostly isn't

What actually happens up there

Less oxygen per breath means less oxygen in your blood, and your body compensates by breathing faster and beating faster. At night that compensation becomes unstable: breathing overshoots, oxygen rises, the drive to breathe falls away, breathing pauses, oxygen drops, and the cycle repeats. It is called *periodic breathing*, and above about 4,000 m it is not a disorder — it is what a healthy respiratory system does when the feedback loop it runs on gets twitchy.

Insalaco and colleagues monitored nine climbers overnight at Everest North Base Camp, 5,180 m. Periodic breathing was present in every single one of them. On the first night, waking oxygen saturation averaged 77.4%. Their heart rate during those breathing cycles swung between roughly 54 and 72 beats a minute.

And what acclimatisation looks like in numbers

This is the part worth knowing, because it is visible in your own data. By the tenth night at the same altitude, the same climbers had waking saturation of 82.5% — five points better — and during sleep with regular breathing it rose from 73.3% to 77.8%. Heart rate went the other way, dropping from about 72 to 63 at the top of each breathing cycle.

Oxygen up, heart rate down, over roughly a week and a half. That is acclimatisation, and it is the clearest thing a wearable can show you on a mountain: not the absolute numbers, but the direction they move over successive nights.

Now the honest caveat about those numbers

Nine elite climbers at 5,180 m is not you at 1,800 m in the Alps. That study measured a population and an altitude far beyond a normal holiday, and the magnitudes do not transfer — you will not see saturation in the 70s on a ski trip, and if you do, that is not a data-interpretation question, that is a doctor question.

What transfers is the shape: saturation lower than your sea-level normal, heart rate higher, sleep more broken on arrival, and all three improving across the first several nights. Read the direction, not the value.

What your ring will show, and what to ignore

Expect a low blood-oxygen reading and do not treat it as a malfunction or a diagnosis — a ring measures saturation through the finger by optical absorption, and a genuinely lower saturation is exactly what it should report at altitude. Expect resting heart rate elevated for several nights. Expect deep sleep and REM to take a hit on night one, and expect more awakenings than you remember having.

The thing to ignore is the readiness score. Every input it is built from is legitimately depressed by being at altitude, so it will tell you that you are wrecked for several mornings in a row. That is not new information about you; it is a restatement of your altitude. A score is a summary of numbers that assume you are where you usually are.

The part that is worth attention

Two things. First, the trend across nights: if saturation is still falling and heart rate still climbing on night four, you are not acclimatising, and that is genuinely worth acting on by going down rather than up. Second, anything that does not fit the pattern — a fever, a cough, a headache that will not clear, breathlessness at rest. Altitude illness is a clinical matter with a well-understood answer, and no wearable diagnoses it.

And when you come home, expect a few days where everything looks unusually good against a baseline your body built at sea level. That is the same effect running backwards, and it is equally uninformative.

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