A minority of people, about 20%, have some symptoms of altitude sickness if they ascend to about 2500 meters (8000 feet) above sea level and sleep there. However, most people will acclimatize to 3000 meters (10,000 feet) with relative ease, perhaps having symptoms after the first night.
Acclimatizing to heights of 3000–5000 meters (10,000–16,000 feet) is much more difficult, and it is here that it is absolutely necessary to ascend slowly and return to a lower altitude to sleep if you have been travelling around at a higher altitude during the day. Over 50% of people will become ill if they ascend rapidly from sea level to 3500 meters (11,000 feet) without acclimatization, and everyone will if they ascend rapidly to 5000 meters (16,000 feet).
