Yosemite's high country is breathtakingly beautiful. Sometimes that's not just an opinion, but it becomes literally true. High altitudes cause problems for visitors who have not yet become acclimatized to the park's mountains. This typically occurs when visiting elevations above 8000 feet, for example, along Glacier Point Road or Tioga Road in the Tuolumne Meadows area.
There are several things you can do to help avoid experiencing the most common problems at altitude. First, be careful not to over-exert yourself until you've become acclimatized. Drink enough water and avoid alcohol use. Get plenty of rest, and try to sleep at either the same or higher elevations than you'll be hiking at the following day.
Commonly known as "altitude sickness," Acute Mountain Sickness (AMS) affects some people more than others. Even if you've had zero problems at altitude before, that doesn't mean you'll be immune to the effects of high elevations every time. Common symptoms of AMS include fatigue, dizziness, shortness of breath, headaches, nausea and insomnia.
If these symptoms do not resolve reasonably fast, descend to lower elevations. Don't ignore any severe signs or symptoms for long. Left untreated, AMS can progress to more serious, life-threatening conditions, such as high-altitude cerebral edema (HACE) or high-altitude pulmonary edema (HAPE), although these usually only affect backpackers and mountaineers.