Long ago I used to climb mountains - real climbing, not just backpacking and hiking. Coming from near sea level in Texas, I was taught that once above about 8,000 feet to spend one day per 1,000 feet of elevation gain for acclimation. You can go somewhat higher during the day for a few hours, but should spend the night moving up about 1,000 feet at a time. You body is undergoing changes in response to the elevation, both with respect to decreased levels of oxygen, decreased air pressure, decreased humidity, and increased levels of light. As others have mentioned, pay attention to assure you are well hydrated. If you are hiking and carrying a load, one gallon a day is not unusual. Watch your urine for color change. Clear is preferable, yellow is time to drink up. Anything darker, stop your physical activities and make rehydration a priority. Also, watch for signs of edema - mild swelling of the feet is tolerable. Swelling of the ankles is a warning to move down. If you hear "crackles" in your lungs while breathing, move down immediately. Fluid accumulating in your lungs is a serious problem requiring priority action. Headaches can be a symptom of lots of things from inadequate conditioning, aching back musles and spasms, dehydration, inadequate time to acclimate, or from cerebral edema i.e. fluid accumulation inside of the skull. If vomiting begins, move lower for a day and make sure you are adequately hydrated. Also, boost your red meat intake about a week before going, and up the intake of dark, green vegetables to boost your iron levels, which aid in oxygen uptake. Finally, go slow and easy. Hiking and carrying a load on a steep slope, 1 m.p.h. is pretty normal and a good pace. Rest often, drink often, and take some photos.
