How altitude sickness affects you can range from mild to severe and occurs because less oxygen is reaching the muscles and the brain, requiring the heart and lungs to compensate by working harder. Symptoms of Acute Mountain Sickness (AMS) usually develop during the first 24 hours at altitude but may be delayed up to 3 weeks. Mild symptoms include headache, lethargy, dizziness, difficulty sleeping and loss of appetite. AMS may become severe without warning. Please consult your doctor before departure to ensure you have an adequate level of fitness and health.