When climbing Mount Everest, low air pressure in the high altitudes means less oxygen. Once a climber reaches 10,000 ft, they must remain at that height for three days, so their body acclimates to the decrease in oxygen. Many climbers can suffer from disturbed sleep, headaches and dizziness, though these symptoms generally subside within three days. Severe acute mountain sickness can occur when a climber ascends too quickly because a lack of oxygen causes fluid to leak from the blood vessels into the brain or the lungs. This eventually causes shortness of breath, confusion, loss of co-ordination, and hallucinations.