A small mountaineering tent glowing warm amber at night on a high snowy ridge beneath dark peaks and a star-filled sky
| |

How Does Altitude Affect Sleep? The Thing Everyone Blames Is Not the Cause

ByBogdan Bilt
Quick answer: Sleep starts to suffer from around 2,000–2,500 m and is reliably disrupted above 3,000 m, where almost every healthy visitor begins breathing in cycles at night — a run of deep breaths, a pause, then a jolt. At 4,300 m deep sleep roughly halves and it takes twice as long to drop off. Every guide on the internet blames the cyclical breathing for this. In 2024 a randomised trial abolished that breathing and measured the sleep anyway — and the sleep was just as broken. Which means the thing most advice is aimed at is not the thing doing the damage.

If you have ever arrived in Cusco, or La Paz, or a trekking lodge in Nepal, you will know the specific flavour of it. You are exhausted. You go to bed early. And then you lie there, heart going, dropping off and surfacing, dropping off and surfacing, and in the morning you feel as though you were awake all night and someone has filled your head with sand.

What follows is what the measurements actually show, including one result that should change the advice and has not yet filtered through to most of the pages you will find on this.

What actually changes, in minutes

The cleanest way to see it is a study that took the same ten lowlanders, ran full overnight polysomnography on them at sea level, then did it again at 4,300 m. Nobody is estimating here, and nobody is comparing one group of people with a different group. It is the same ten people, twice.

One night, ten lowlanders, 4,300 m

The same people measured at sea level and again at altitude. Bars are drawn to scale against the sea-level night.

Sea level — 433 minutes asleep

80
273
81

4,300 m — 412 minutes asleep

45
297
70

Deep sleep    Light sleep    REM  —  figures in minutes

Total sleep barely moved: 433 minutes down to 412, a loss of about twenty minutes. That is not the story. The story is what happened inside those hours:

  • Deep sleep fell from 80 minutes to 45 — a drop of 43%.
  • Time to fall asleep nearly doubled, from 11.6 minutes to 22.2.
  • REM fell from 81 minutes to 70.
  • Light sleep rose to absorb the difference, from 273 minutes to 297.

So you spend almost as long in bed asleep, and get roughly half the deep sleep out of it. That is why the arithmetic of “I got seven hours” does not match how you feel. You did get seven hours. They were the wrong seven hours.

The breathing everyone describes

The other thing that happens, and the thing your tent-mate will complain about, is periodic breathing. You take a run of progressively deeper breaths, then your breathing stops for several seconds, then it starts again with a gasp. The cycle repeats, over and over, for much of the night.

This is not rare and it is not a sign that something has gone wrong with you. Above roughly 3,000 m, almost all healthy people do it, most heavily in non-REM sleep. In the 4,000 m trial we are about to come to, periodic breathing occupied 54% of total sleep time on the first night.

The mechanism is a control problem rather than a blockage. Low oxygen makes your breathing system hypersensitive to carbon dioxide. You over-breathe, blow off too much CO2, and your body briefly loses the signal telling it to breathe at all — so it stops, CO2 climbs back, and it restarts hard. The loop overshoots in both directions all night. It is worth being clear that this is central, not obstructive: nothing is closing in your throat.

Two other details worth knowing. It largely belongs to visitors — people who live at altitude long-term show it far less. And your oxygen levels are at their lowest of the entire day while you are asleep, not while you are hauling uphill with a pack on.

The experiment that broke the explanation

Here is where almost every article on this subject, including the ones currently ranking above this one, states something that has now been tested and did not hold up.

The standard account runs: low oxygen causes periodic breathing, periodic breathing fragments your sleep, therefore your sleep is bad. It is tidy, it is mechanistically plausible, and until recently nobody had separated the two halves of it.

In 2024 a team working across Innsbruck, Bolzano and Zurich did exactly that. Twelve men spent three nights at a simulated 4,000 m, twice, four weeks apart, with full polysomnography. The trick was a canopy over the sleeper’s head that retained their own exhaled CO2. Raising inspired CO2 to about 1.76% closes the control loop and stops the overshoot — and crucially, the sleeper cannot tell which night is which, so it works as a genuine placebo-controlled crossover.

It worked, emphatically, on the breathing:

  • Time spent in periodic breathing fell by 38 percentage points.
  • The apnoea–hypopnoea index fell by 58 events per hour.
  • Oxygen desaturations fell by 56 per hour.
  • Average oxygen saturation across the night rose by 2%.

By any measure, the periodic breathing was abolished. And the sleep?

Essentially unchanged. The only difference the researchers could find in sleep architecture was a slight increase in deep sleep on the third night, scraping in at P = 0.045. Their conclusion, in their own framing: periodic breathing does not explain the deterioration in sleep commonly seen at high altitude.

Take that seriously for a moment. The most visible, most dramatic, most complained-about feature of sleeping at altitude — the thing that wakes your tent-mate up — turns out to be largely a passenger. Remove it and you still sleep badly. The hypoxia appears to be doing the damage directly, by some route that does not run through your breathing pattern.

This is one study, in twelve men, in a hypobaric chamber rather than on a mountain, and it deserves replication before anyone treats it as settled. But it is a properly controlled test of the specific claim everyone else is making, and it came back negative.

What that leaves you with

If the breathing is not the lever, the lever is oxygen and time. Which narrows the practical list considerably, and quietly explains why so much altitude sleep advice disappoints.

Ascent rate is the one you control. Nothing you do in the tent competes with how fast you got there. The old climbing maxim — climb high, sleep low — is really a statement about the altitude your body has to cope with for eight unconscious hours, which is the hardest part of the day for it.

Do not schedule your hardest day before the night you most need to sleep. This one is counterintuitive and well tested. Over ten days at a simulated 4,175 m, people doing two hours of moderate cycling a day had more apnoea–hypopnoea events, more light sleep and more shifting between sleep stages than matched controls who did nothing. Exercise at altitude destabilises breathing further rather than earning you a deeper night.

Acetazolamide is a conversation with a travel-medicine doctor, not a supplement. It is the one intervention with a clear mechanism here: it produces a mild metabolic acidosis, which keeps you breathing steadily and lifts your oxygen levels overnight. It genuinely improves sleep and altitude-sickness symptoms. It is also a prescription drug with real side effects, and the decision belongs to someone who knows your history — arranged before you travel, not negotiated at a pharmacy counter at 3,600 m.

Be sceptical of the gadget answers. Elevating the head of the bed is standard advice on most pages about this, and it is sound guidance for obstructive apnoea and for reflux — but the problem at altitude is central, and we could not find a trial testing head elevation at altitude at all. If you sleep better propped up, do it. Just do not expect it to touch the mechanism.

When it is not just the altitude

Bad sleep at altitude is normal. The symptoms below are not, and they are not a sleep problem.

Descend, and get help. Headache with vomiting that will not settle. Confusion, or someone who cannot walk a straight line. Breathlessness while sitting still, or a cough with frothy or pink sputum. These point to acute mountain sickness tipping into high-altitude cerebral or pulmonary oedema, both of which are emergencies and both of which are treated by going down. Never give a sedative to someone with these signs to help them sleep through it.

Common questions

At what altitude does sleep start to get worse?

Changes are measurable from around 2,000 to 2,500 m, which is why people notice it in ski resorts and not just on expeditions. Above about 3,000 m, periodic breathing during sleep is close to universal in healthy visitors. By 4,300 m, deep sleep is down by roughly 43% and it takes about twice as long to fall asleep.

Is periodic breathing at altitude the same as sleep apnoea?

No, and the difference matters. Obstructive sleep apnoea is a blockage — the airway closes while you keep trying to breathe. Altitude periodic breathing is central: nothing is blocked, the instruction to breathe simply switches off for a few seconds because the control loop has overshot. It is close to universal above 3,000 m in visitors, far less common in people who live at altitude, and it resolves when you come down.

Does it get better if I stay up there longer?

Partly, and less than you would hope. How your sleep feels, and its overall structure, do tend to improve over several days as you acclimatise. The breathing disturbance is another matter: in one 21-day study at around 4,000 m equivalent, the apnoea–hypopnoea index roughly doubled between the first night and the twenty-first. “Give it a few days” is reasonable advice about how you will feel, and not a safe assumption about what your breathing is doing.

Should I take a sleeping tablet at altitude?

Ask a doctor before you travel. Low-dose temazepam and zolpidem have been reported to improve sleep at altitude without worsening the breathing pattern, which is reassuring as far as it goes. But sedatives and low oxygen are a combination to be deliberate about rather than casual with, and the drug with the best case for itself here — acetazolamide — is the one that treats the underlying problem rather than the symptom. None of this is a decision to make on the mountain.

Why do I feel worse in the morning than when I went to bed?

Because your oxygen levels are lowest while you are asleep, not while you are climbing. A night at altitude is the most hypoxic stretch of your day, and you spend it unconscious and unable to do anything about it. That is also why acute mountain sickness so often announces itself on waking.

Sources

Research located via PubMed. This is general information about a well-documented physiological effect, not medical advice, and it is not a substitute for a travel-medicine consultation before you go somewhere high.

Altitude is rarely the only thing you changed that week

Most people who arrive somewhere high have also crossed several time zones to get there, and the two problems feel identical at 3am while needing completely different fixes. A week of diary data separates them: sleep that drifts later each night is a clock problem, while enough hours with no benefit from them is not.

Free sleep diary →  ·  The jet-lag guide →  ·  How people sleep in Antarctica →  ·  How submariners sleep →