Altitude acclimatization: what’s best?

Adaptation or acclimatization to altitude is always good for a hefty debate. I’ve been in places where it’s been told that we should go “up” during the day, and down for the night. Or the other way around. Or simply stay on the mountain.

The thing is: it really depends on your goals.

In this blog post I will first dig in to the need for acclimatization. Then I will go in (sort of) details about what adaptations the body actually makes when exposed to high altitudes.
Lastly, I will dive in the chapter of “methods”: what is the best way to get your body used to higher altitudes? And what is considered to be a high altitude?

As I am not a medical scientist myself: all material in this post is cited and paraphrased from the sources in the footer of the article. The sources are academic literature: a result of scientific studies. Not a result from anecdotal evidence – unless clearly noted as such!

The need for altitude acclimatization

The human body (or any other body) is a beautiful product of millions of years of evolution. It’s designed to perform at it’s best in the surroundings where it is. The body is totally tuned to preserve homeostasis: keep all processes balanced. If you exercise, the heart rate goes up to be able to pump more blood to the muscles. More oxygen is delivered and this needs to be provided by the lungs. When exercise is over, the body still works to get rid of excess heat to return to its original situation: homeostasis is restored.

This is not just the case with exercise. Also with cold, digestion, or the situation in this blog post: altitude increase.
At sea level, there is about 21% oxygen in the air. We need oxygen to release energy: Oxygen (O2) interacts with our fuels (proteins, fats and carbohydrates) to generate for example movement. The result of this interaction is movement, heat and CO2: carbon dioxide. This carbon dioxide is released into the air in your lungs and replaced by oxygen. In that order: our respiratory system is primarily CO2-driven and not oxygen driven.

This oxygen level is quite stable, regardless of elevation. That is however in relative terms. In absolute terms, the amount of oxygen decreases. But also the pressure on your lungs decreases: the pressure of the surrounding gas (air) is a big driver of the gas exchange in your lungs. Effectively, 2 things happen: this air pressure is lower and the absolute availability of oxygen is lower. (1)

As your body is made to perform best in its natural habitat, it needs to adapt to this new situation. If it doesn’t, you enter a phase of hypoxia and develop altitude sickness.

Acclimatization, then, is a series of physiological responses to hypoxia that serve to offset hypoxemia, improve systemic oxygen delivery, and avoid AMS and HACE.

Imray et al. (2)

Bear in mind the difference between hypoxia (reduced level of tissue oxygen) and hypoxemia (decrease in the partial pressure of blood oxygen).

Body responses to high(er) altitudes

The primary function of all body adaptations lie in the preservation of tissue and organs.
As stated before: the human body is a marvellous result of adaptation. It can adapt to a new situation literally in a matter of minutes (1).

The first adaptation is acute: an increase in breathing frequency and heart rate. The stroke volume decreases slightly. After about 10 days, the heart rate is fully back to normal. The largest adaptation is within the first 10 minutes. This coincides with the increase in breathing frequency. As breathing frequency increases, the heart rate calms down.

This increased breathing frequency is actually “hyperventilation”. The breathing is increased to get more oxygen in the blood stream – which is different from the normal breathing pattern which has the function to remove CO2 from your blood (and the replenishment with oxygen is a result, not the primary task). You might think “so what” – but actually you need carbon dioxide in your blood as it part of the mechanism that controls the acidity of your blood.

But the body adapts: after 90-100 minutes, the so-called CO2-ventilator response kicks in and restores the level of CO2 in your blood stream.

After about 1 day, the body starts to decrease the blood plasma volume which increases the (relative) number of red blood cells. This is also a sign to the body that more red blood cells need to be produced: the all-to-famous EPO which is naturally decreted by your liver.
This smaller volume of blood plasma does result in thicker blood.

It takes 10-15 days before the next major adaptation starts to occur: an increase in capillary density. It’s the combination of increased Hb-levels (and thus red blood cells) and increased capillary density that are responsible for increased physical performance. In other words: a couple of days at high altitude does not affect your fitness level.
Do not anticipate on a new personal record on a 10k-run after climbing Mont Blanc!

What is the best way to acclimatize?

Sleeping high, and exercising low. Or climb high and sleep low. Or stay high. What is the best?

Science can provide the answer – but it really depends on your goals. Acclimatization is primarily focused on preventing altitude sickness. But for (elite) athletes, the goal is to perform better at sea-level.

Until approximately 2500 meters altitude, there is no real issue with performance or adaptation. Oxygen levels in the air are lower, but your body is well-able to compensate by increased breathing and heart rate. Above 2500 meters, and in some sources above 3000 meters (2,3), pre-cautions need to be taken to prevent altitude sickness.

To stay on the safe side: be cautious between 2500 and 3000 meters. And use the best practices from studies to prevent mountain sickness: climb 300 meters per day and descent 500 meters when symptoms occur.

What does this mean in practice? What does it mean for “climbing high and sleeping low”? Well: it doesn’t give you any proven benefit to climb high and sleep low. Gaining more altitude doesn’t necessarily give you an advantage. Climbing high and sleeping low is actually lost in translation. Many people think that it means starting in the valley, climb as high as possible, and sleep again in the valley. That’s not what is meant. It simply means: sleep at a safe altitude: no more than 300 meters above your previous resting place!

Climbing to 3300 meters from 3000 meters is, in terms of adaptation, the same as climbing from 3000 to 3600 and descending to 3300 meters. But don’t stay overnight at 3600 meters: do that the next day. You can climb safely every day 300 meters and add an extra rest-day each 1000 meters.

Make sure to not drink alcohol, as alcohol prohibits the first adaptation of heart rate and breathing frequency.

For increasing performance at sea level: think differently

It is, however, very different when you want to increase performance (4). In the study from Levine & Stray-Gundersen, it’s been described what is the best way to adapt your body to have the best sea-level performance.

For this study, they sent people to altitudes that are not prone to cause mountain sickness of any kind. It’s found that the largest increase in performance was gained by the athletes that exercise in the valley, but sleep at high altitude (2500m).

Athletes that did their exercise at high altitude and slept at high altitude, did see a larger increase in red blood count & Hb-levels in the blood – however their performance did not increase as much as with the athletes that were sleeping in the valley. This is not primarily due to a lack of acclimatizing: in fact their body adapted better. One of the adaptations is lowering the maximum heart rate at altitude: a natural brake to performance which isn’t present in the group that slept in the valley.

This does show that this kind of adaptation is the exact opposite of the climbers mantra: the athletes all slept at high-altitude and exercised in the valley.


To conclude this blog post and summarize:
High-altitude adaptation is necessary to prevent mountain sickness. It doesn’t increase your physical performance at sea level – unless you’ve spent weeks at high altitude.

Safe climbing is gaining approximately 300 meters per day – from camp-to-camp (or refuge to refuge or wherever you sleep). There is no problem in gaining more meters per day (within boundaries) and descending on the other side to sleep at the appropriate elevation. Don’t use alcohol until acclimatized.

For most hikes, altitude isn’t the biggest issue. I do say “most” – as there are many exceptions. Below picture is taken at High Camp on the Annapurna Circuit. It’s 700 meters higher than the previous camp – and 600 meter below the 5416 meter Thorung La pass. There are no intermediate rest-points…

High Camp, Annapurna Circuit


  1. Peacock, A. J. (1998). Oxygen at high altitude. Bmj317(7165), 1063-1066.
  2. Imray, C., Wright, A., Subudhi, A., & Roach, R. (2010). Acute mountain sickness: pathophysiology, prevention, and treatment. Progress in cardiovascular diseases52(6), 467-484.
  3. Hackett, P. H., & Roach, R. C. (2001). High-altitude illness. New England Journal of Medicine345(2), 107-114.
  4. Levine, B. D., & Stray-Gundersen, J. (1997). “Living high-training low”: effect of moderate-altitude acclimatization with low-altitude training on performance. Journal of applied physiology.

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