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Physiology: what changes occur in the first 72 hours at altitude?

Published 31 December 2025 · 4 min read

The first 72 hours at altitude are physiologically decisive. During this short period, your body makes the biggest adjustments to the lower oxygen supply. How you get through this phase largely determines whether you feel comfortable, sleep well and can continue your ascent safely, or whether you develop symptoms such as headaches, insomnia and altitude sickness.

In this article, you will discover:

what there is by phase (0–24, 24–48 and 48–72 hours) changes in your body
which adjustments are desirable and which signals are warning signs
why these first few days often go wrong
how you can support these physiological processes
what this means for acclimatisation, pace and pre-acclimatisation

Key message: Your body can adapt very well to altitude, but only if you respect your pace, recovery and sleep.

Why the first 72 hours are so crucial

As soon as you are above approximately 2,500–3,000 metres When you breathe in, the oxygen pressure drops noticeably. Each breath contains fewer oxygen molecules, while your energy consumption remains the same or even increases due to cold, exertion and stress.

Your body does not react to this all at once, but in phases. The first 72 hours are characterised by rapid, largely neurohormonal adjustments. Structural adjustments (such as extra red blood cells) follow later.

This explains why many people:

day 1 feeling “fine”
day 2 sleeping worse
day 3 suddenly develop symptoms

Phase 1: the first 0–24 hours at altitude

What happens immediately?

Within minutes to hours after arriving at altitude, your body reacts with:

faster breathing (hyperventilation)
higher heart rate, even at rest
increase in stress hormones (such as adrenaline)

These are acute compensatory mechanisms to compensate for the lower oxygen supply.

What do you notice yourself?

out of breath more quickly during exercise
slight feeling of unease
often still few complaints

Many people underestimate this phase because they “still feel fine”. That is precisely why mistakes are often made here.

What helps at this stage?

very slow pace (even if it feels easy)
drink enough, but do not overdo it
no strenuous effort
do not sleep higher than planned

Phase 2: 24–48 hours at altitude

Ventilatory adaptation begins

During this phase, your body attempts to make oxygen uptake more efficient by:

further increased breathing rate
changes in acid-base balance (respiratory alkalosis)
increased urine output (fluid loss)

This is physiologically necessary, but may cause side effects.

Typical experiences in this phase

sleep worse or waking up frequently
mild headache
reduced appetite
feeling tired more quickly

This is often the moment when people start to wonder: “Will this pass, or will it get worse?”

What does physiology say?

Mild symptoms at this stage include not necessarily dangerous. They are often part of normal acclimatisation. The risk arises when you, during this phase:

rising too quickly
does not get enough sleep or rest
ignores signals

Phase 3: 48–72 hours at altitude

Decisive phase for acclimatisation

During this period, there are roughly two scenarios:

Scenario A: successful adaptation

breathing stabilises somewhat
sleep becomes more restful
energy is partially recovered
mild symptoms are decreasing

Scenario B: failed acclimatisation

persistent or increasing headache
nausea
obvious sleep problems
feeling of “running on empty”

This is often the moment when altitude sickness manifests itself if the pace is too high.

What's happening under the bonnet?

Start slowly:

kidney adjustments to correct acid-base balance
signals for increased EPO production
adjustment of muscle and cerebral blood flow

But these processes take time. You cannot force them.

Why these 72 hours often go wrong

The most common mistakes:

“I feel good, so I can keep going.”
skip acclimatisation days
let the group or schedule determine the pace
underestimating sleep and recovery

Physiologically speaking, you are then playing catch-up. Complaints often arise. delayed.

What does this mean for acclimatisation strategy?

1) Respect sleeping height

Your body recovers during sleep. Sleeping too quickly is the biggest risk factor.

2) Use acclimatisation days effectively

Active rest (climb high, sleep low) supports physiological adaptation without overexertion.

3) Do not expect a linear process

Good days and bad days alternate. The trend is more important than the snapshot.

The role of pre-acclimatisation

Because the first 72 hours are particularly stressful, more and more hikers and athletes are opting for pre-acclimatisation.

By sleeping in an altitude tent beforehand:

the ventilatory response is already partially active
the first night at real altitude is calmer
the physiological shocks are smaller

This does not replace acclimatisation on the mountain, but it reduces the stress on your system.

Further explanation:
Altitude tent hire and personal advice

FAQ: physiology and the first days at altitude

Is it normal that I sleep poorly?

Yes. Sleep disturbances are one of the most common reactions in the first 48 hours.

When should I be concerned?

If symptoms increase after 48–72 hours or worsen with further elevation.

Does fitness help?

Good physical condition helps with exertion, but does not protect against altitude sickness.

Conclusion: what changes occur in the first 72 hours at altitude?

The first 72 hours at altitude are a physiological test phase. Your body is shifting to a new equilibrium, but that takes time and rest.

Those who respect this period:

acclimatises more safely
sleeps better
has more energy left
reduces the risk of altitude sickness

Those who push themselves too hard during this period often pay the price later on.

Would you like to be well prepared and get through these first 72 hours more effectively?
Ask for personal advice or hire a height tent

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