The higher one climbs, the less oxygen there is in the air. When people don’t get enough oxygen, they often begin to gasp for air. They may also feel dizzy and light-headed. Besides these common symptoms of altitude sickness, others such as nausea, headache, tiredness, and difficulty sleeping may also occur. At heights of over 18,000 feet, people may be climbing in a constant daze. This state of mind can have a serious effect on their judgment.