The problem with high elevation is that there is less atmospheric pressure and consequently less oxygen gets to the lungs. To compensate the body increases respiration rate; however, this also increases water loss as vapor escapes the lungs. In most people, the lack of oxygen and combined dehydration causes headaches, fatigue, upset stomach, dizziness, and/or disturbed sleep.
Eventually the body copes by producing more red blood cells. Having more cells in your blood stream means more oxygen is carried to alleviate your oxygen-deprived cells. Symptoms then start to disappear within a week or two. Unlike us humans, the high elevation doesn't pose a problem for the locally adapted wildlife.