Acclimatisation Tips for the Everest Base Camp Trek

Acute Mountain Sickness affects anywhere from roughly a quarter to over half of trekkers who ascend too quickly on the Everest Base Camp route, depending on the study and the checkpoint measured, but it is largely preventable with a properly paced itinerary and basic self-monitoring. This is an entirely different risk category from the mountaineering death zone above 8,000 m, more than 2,600 vertical metres above anywhere this trek goes.

Acclimatisation is the physiological process by which the body adapts to reduced oxygen at altitude, primarily through a faster breathing rate and, over several days, a rise in red blood cell production driven by the hormone erythropoietin. The Everest Base Camp Trek's mandatory rest days exist specifically to give this process time to work before ascending further.

• Ascent rule: no more than 300-500 m of net sleeping elevation gain per day above 3,000 m.
• Rest days: one for every 900-1,000 m gained, which is why every responsible itinerary stops at Namche (3,440 m) and Dingboche (4,410 m).
• What actually counts: sleeping altitude, not the highest point reached during the day's walk.
• Diamox: 125 mg twice daily, started one to two days before ascent. A 2021 meta-analysis of 22 trials found a 48% relative risk reduction in AMS at that dose.
• Hydration: at least 3-4 litres of water daily, and no alcohol above 3,000 m.
• When to descend: a Lake Louise Score of 6 or higher, or any sign of ataxia or confusion.

What is the 300-500 m ascent rule?

Above 3,000 m, the trekking medicine consensus is to gain no more than 300-500 m of net sleeping elevation per day, and to build in a rest day for every 900-1,000 m gained. This is why every responsible EBC itinerary includes acclimatisation days at Namche (3,440 m) and Dingboche (4,410 m). Skipping either to save a day is the single most common cause of AMS on this route.

What do the Wilderness Medical Society guidelines say?

The Wilderness Medical Society's 2024 clinical practice guidelines for acute altitude illness, authored by Andrew Luks and colleagues, give the 300-500 m rule its precise formal source: avoid a first sleeping altitude above 2,750 m in a single day, ascend no more than 500 m per night in sleeping elevation once above 3,000 m, and add one extra acclimatisation night for every 1,000 m of sleeping-altitude gain. The guidelines specifically note that sleeping altitude, not the highest point reached during the day's walk, is what the body's adaptation responds to, which is exactly why the classic itinerary's acclimatisation-day hikes work: climbing to Everest View Hotel (3,880 m) from Namche during the day, then returning to sleep at 3,440 m, delivers the altitude stimulus without demanding the body adapt to sleeping there yet.

What does climb high, sleep low mean?

On rest days, the best acclimatisation strategy is to hike higher during the day, to the Everest View Hotel (3,880 m) from Namche, or Nangkartshang Peak (5,083 m) from Dingboche, then descend back to the lower sleeping elevation. This exposes the body to reduced oxygen without requiring it to sleep there yet.

Why sleep feels different above 3,000 m

Periodic breathing, a cyclical pattern of deep breaths followed by several seconds of shallow breathing or brief pauses, develops in nearly all healthy trekkers sleeping above roughly 3,000 m and is a normal, expected response to lower oxygen rather than a warning sign on its own. It happens because low blood oxygen triggers hyperventilation, which then drops carbon dioxide below the threshold that normally keeps the drive to breathe steady, producing a repeating 15 to 20 second cycle. This is a different, more benign pattern than Cheyne-Stokes respiration, a longer, minute-scale cycling seen in certain heart conditions, and it typically eases within a few nights as acclimatisation progresses. Waking briefly gasping for air once or twice a night at Dingboche or Lobuche is common and not itself a symptom to report on a Lake Louise Score; a full night of genuinely disrupted sleep alongside headache or nausea is a different, more meaningful signal.

How does Diamox work, and how should you take it?

Acetazolamide, sold under the brand name Diamox, is the standard prophylactic medication for AMS prevention on this route. It works by making the kidneys excrete bicarbonate, which acidifies the blood slightly and triggers faster, deeper breathing, the same adaptation the body would otherwise take days to build naturally. The commonly recommended dose is 125 mg twice daily, started one to two days before ascent and continued for two days after reaching the highest sleeping altitude of the trek. A 2021 systematic review and meta-analysis in the journal Wilderness & Environmental Medicine, pooling 22 randomised controlled trials, confirmed a 48% relative risk reduction in AMS incidence at the 125 mg twice-daily dose specifically, with the trial-sequential analysis behind that review concluding the evidence base is now large enough to consider the effect firmly established rather than provisional. Common side effects include tingling in the fingers and toes and a metallic taste in carbonated drinks, both harmless, along with increased urination, which makes the hydration target below even more important. Diamox is a sulfonamide-derived drug, so trekkers with a genuine sulfa drug allergy need to discuss an alternative with a doctor well before departure rather than discovering the reaction on the trail. It remains a prescription medication in most countries, so this conversation belongs with a doctor before departure, not a decision made on the trail.

How much should you drink, and what should you track?

Drink at least 3-4 litres of water daily; dehydration mimics and worsens AMS symptoms. Avoid alcohol above 3,000 m, and eat enough carbohydrate-heavy food even if appetite drops, since appetite loss is itself a mild altitude symptom. A pulse oximeter, a small clip-on device measuring blood oxygen saturation, is a useful trail companion, though it supplements symptom tracking rather than replacing it: readings vary enough between individuals at the same altitude that one number rarely predicts AMS risk on its own, and a downward trend over several days matters more than any single reading. Track symptoms using the Lake Louise Score:

• 3 to 5, with headache present: mild-to-moderate AMS. Stop ascending.
• 6 or higher, or any ataxia or confusion: descend immediately, regardless of the itinerary schedule.

Which personal risk factors matter beyond pace?

Ascent rate is the biggest lever any trekker controls directly, but it is not the only variable. A personal history of AMS on a previous trip at altitude is one of the more consistently identified risk factors in the research, meaning trekkers who have felt genuinely unwell above 3,500 m before should plan an even more conservative pace than the standard itinerary allows, not just repeat the same schedule. Genetics play a role too: individual susceptibility varies enough between otherwise similarly fit trekkers that AMS is not a reliable proxy for fitness level, and a strong trail runner can feel worse at 4,400 m than a slower, more cautious hiking partner. None of these factors are something a trekker can change before departure, but knowing your own history changes how conservatively you should treat the itinerary's built-in rest days, whether that means treating them as fixed rather than optional, or discussing an even slower custom pace with your operator in advance.

When acclimatisation isn't working: HACE and HAPE

AMS, HACE, and HAPE sit on the same spectrum of altitude illness, and recognising which one is happening changes the correct response. AMS causes headache along with nausea, fatigue, or dizziness, and is the only one of the three considered mild to moderate. High Altitude Cerebral Edema (HACE) is brain swelling that typically appears three to five days after a significant ascent, marked by ataxia (a loss of coordination visible in something as simple as walking a straight line), confusion, and drowsiness that can progress toward coma without immediate descent. High Altitude Pulmonary Edema (HAPE) is fluid in the lungs, often appearing without any preceding AMS symptoms, marked by breathlessness at rest, a persistent cough, and in advanced cases blood-tinged sputum. Both HACE and HAPE are medical emergencies requiring immediate descent regardless of distance already covered, a call no guide should hesitate to make. What that decision actually looks like on the trail, and what happens afterward, is covered in full in our guide to turning back due to altitude sickness.