Effects of Altitude

Altitude sickness may develop in otherwise healthy and fit people who are exposed to rapid increases in altitude. It can develop at altitudes as low as 8,000 feet (Yosemite Valley's elevation is 4,000 feet). The risk of occurrence increases with age and with diseases of the heart and lungs. 

The symptoms include headache, nausea, insomnia, irritability, shortness of breath, general malaise and fatigue. The best way to avoid it is to slowly acclimatize yourself to higher elevations, over the span of two to three days by gradually gaining elevation until you reach 10,000 feet (Tioga Pass). Avoid alcohol, sugar and high-fat meals. 

Should altitude sickness develop, descend to a lower elevation. The Yosemite Medical Clinic in Yosemite Valley is experienced in diagnosing and treating this sickness.