Acclimating to high altitude takes time, and the single most important rule is to ascend gradually. Your body needs anywhere from a few days to several weeks to adjust to reduced oxygen levels, depending on how high you go and how quickly you get there. Rushing the process is the primary cause of altitude sickness, which affects up to half of travelers who climb above 8,000 feet without proper preparation.
What Happens to Your Body at Altitude
At higher elevations, each breath contains less oxygen. Your body responds immediately by breathing faster and deeper to compensate. On the summit of Mount Everest, for example, ventilation increases by a factor of five compared to sea level. Even at more modest elevations like 8,000 to 10,000 feet, you’ll notice yourself breathing harder during activity that would feel easy at home.
This faster breathing creates a chain reaction. You exhale more carbon dioxide than usual, which shifts your blood chemistry toward a more alkaline state. Over the first few days, your kidneys work to correct this imbalance, which is part of why acclimatization takes time rather than happening overnight.
Your body also begins producing more erythropoietin, a hormone that signals your bone marrow to make additional red blood cells. More red blood cells means more oxygen-carrying capacity in your blood. But this process is slow: it takes several days before increased red blood cell production is even measurable, and the full adaptation takes weeks to months. This is why elite mountaineers spend extended periods at base camp before attempting high summits.
The Golden Rule: Climb High, Sleep Low
“Climb high, sleep low” is the most widely used acclimatization strategy. The idea is straightforward: you can hike or climb more than 1,000 feet of elevation gain during the day, but you descend to a lower elevation to sleep. This exposes your body to higher altitude during waking hours while giving it the benefit of more oxygen-rich air during the recovery period of sleep.
In practical terms, this means planning your itinerary so that your sleeping elevation increases gradually. Above 8,000 feet, a common guideline is to increase your sleeping altitude by no more than 1,000 to 1,500 feet per night. Every three to four days, build in a rest day where you sleep at the same elevation as the previous night (or lower) to let your body catch up.
Pacing Your First Days at Elevation
If you’re flying or driving directly to a high-altitude destination, the first 24 to 48 hours matter most. Avoid strenuous exercise on your first day. Take it easy, walk around, and let your body register the change. Many ski resorts, trekking towns, and mountain destinations sit between 7,000 and 10,000 feet, which is high enough to cause symptoms if you push too hard too soon.
If your trip allows flexibility, consider spending a night or two at a moderate elevation (5,000 to 7,000 feet) before going higher. Travelers flying from sea level to places like Cusco, Peru (11,000 feet) or La Paz, Bolivia (12,000 feet) are especially vulnerable because there’s no gradual transition. In these cases, plan your first day around rest, not sightseeing.
Stay Hydrated and Fuel With Carbohydrates
You lose water faster at altitude than you realize. The dry mountain air and your increased breathing rate both accelerate water loss through respiration alone, on top of normal sweating. Drink consistently throughout the day rather than waiting until you feel thirsty. A good target is to increase your normal daily fluid intake by at least one to two liters when above 8,000 feet. Pale, clear urine is the simplest indicator that you’re drinking enough.
Your body also benefits from a higher carbohydrate intake at altitude. Carbohydrates produce more energy per unit of oxygen consumed than fat does, which matters when oxygen is scarce. This doesn’t require a radical diet overhaul. It means favoring foods like rice, pasta, bread, oatmeal, and fruit over heavy, fat-rich meals. Many people lose their appetite at altitude anyway, so eating smaller, carbohydrate-rich meals throughout the day tends to work better than three large ones.
Recognizing Altitude Sickness Early
Altitude sickness, formally called acute mountain sickness (AMS), has four hallmark symptoms: headache, nausea, fatigue, and dizziness. Headache is the defining symptom. The standard diagnostic tool used by mountain medicine physicians rates each of these four symptoms on a scale from zero (absent) to three (severe and incapacitating). A score of three or more points, with headache present, indicates AMS.
Mild altitude sickness (a score of 3 to 5) feels like a hangover: a nagging headache, some queasiness, and general sluggishness. Moderate cases (6 to 9 points) involve persistent vomiting, significant fatigue, and dizziness that disrupts your ability to function. Severe cases (10 to 12 points) are incapacitating and require immediate action.
The critical thing to understand is that mild symptoms are common and not necessarily dangerous on their own. They’re a signal to stop ascending. If symptoms don’t improve within a day at the same elevation, or if they worsen at any point, descend. Even dropping 1,000 to 2,000 feet often brings noticeable relief within hours.
When Medication Makes Sense
Acetazolamide is the only FDA-approved medication for preventing altitude sickness. It works by speeding up the kidney’s correction of the blood chemistry changes caused by faster breathing, essentially accelerating the body’s natural acclimatization process. The standard preventive dose is 125 mg taken twice daily.
Most guidelines recommend starting the medication the day before you arrive at altitude and continuing for the first two days there, or longer if you’re still ascending. Some evidence suggests that starting two days before arrival may provide even better protection. Common side effects include tingling in the fingers and toes and increased urination, both of which are harmless but worth knowing about.
Acetazolamide is particularly useful for people who can’t control their ascent rate, such as those flying directly to high-altitude cities, or for anyone with a history of altitude sickness on previous trips. It’s a prescription medication, so plan ahead and talk to your provider before your trip.
Alcohol, Sleep, and Other Practical Factors
Alcohol impairs your breathing response to low oxygen, especially during sleep. Even moderate drinking during your first nights at altitude can worsen symptoms and disrupt already-difficult sleep. Most experienced high-altitude travelers avoid alcohol entirely for the first two to three days.
Sleep itself is often disrupted at altitude. You may wake frequently, experience irregular breathing patterns at night, or feel like you didn’t rest well despite spending enough hours in bed. This is normal and improves as you acclimatize. Sleeping with your head slightly elevated can help.
Physical fitness does not protect you from altitude sickness. Highly fit athletes get AMS at the same rates as sedentary people. Fitness helps you perform better once acclimatized, but it doesn’t speed up the process. The biggest risk factor is how fast you ascend, not how fit you are. Age, interestingly, seems to offer slight protection: older adults tend to get altitude sickness less frequently than younger ones, possibly because they pace themselves more conservatively.
A Sample Acclimatization Schedule
For a trek or climb that goes above 10,000 feet, a sensible approach looks something like this:
- Days 1 to 2: Arrive at a moderate base elevation (7,000 to 8,000 feet). Keep activity light. Hydrate aggressively.
- Days 3 to 4: Begin hiking to higher elevations during the day, but sleep no more than 1,000 to 1,500 feet higher than the previous night.
- Every 3rd or 4th day: Take a rest day at your current sleeping elevation or slightly below.
- Throughout: Monitor yourself for headache, nausea, fatigue, and dizziness. If symptoms appear, hold your elevation. If they worsen, go down.
For travelers headed to a fixed high-altitude destination rather than a progressive climb, the most protective strategy is to build in buffer time. Arrive a day or two early, keep the first day’s plans minimal, and give your body the space it needs to catch up with where you’ve taken it.