Altitude sickness occurs when you go to a high altitude where there is less oxygen than you are used to. Although the concentration of oxygen in the air remains the same (21 percent) at differing altitudes, the number of oxygen molecules per breath goes down as the altitude increases. You have to breathe faster in order to accommodate, and you won't get as much oxygen as you would at a lower altitude.
Symptoms of mild altitude sickness include headache, dizziness, shortness of breath, nausea, weakness and malaise. If you have mild altitude sickness you may have trouble sleeping but if the sickness becomes severe it may actually cause drowsiness. This is a cause for concern, as are the following other symptoms of severe altitude sickness: worsening of the symptoms listed above, persistent cough, clumsiness or lack of coordination, double vision and convulsions. If fluid is leaking from your capillaries into your lungs or brain you may also experience a bubbling sound in your chest and/or confusion due to swelling in your brain. If you experience these symptoms, descend to a lower altitude immediately!
In general you should be able to acclimatize to pretty much any altitude if you give it enough time. It takes one to three days to acclimatize to any given altitude. If you climb slowly and give your body time to adapt you should feel fine and not get altitude sickness. Altitude sickness arises if you go too high to fast or stay up very high for too long. For this reason it's a good idea not to increase your camping altitude (the altitude at which you sleep) by more than 100 feet (300 meters) after you exceed 10,000 feet (3,000 meters) in altitude. There are medications you can take to help you deal with mild symptoms of altitude sickness, but if you experience drowsiness or any of the other symptoms of severe altitude sickness, you must go down to a lower altitude as soon as possible because this is the only cure.