Altitude sickness is the body's response to low oxygen levels at high altitudes, particularly when rapidly moving to altitudes above 8,000 feet, according to WebMD. Symptoms include headache, weakness, difficulty sleeping, nausea and dizziness.
The symptoms of altitude sickness don't always start immediately. Headaches typically begin between two and 12 hours after reaching the higher altitude, states the Centers for Disease Control and Prevention. Some people don't notice the symptoms until the following day.
High-altitude cerebral edema and high-altitude pulmonary edema are more severe forms of altitude sickness, notes the CDC. HACE symptoms include extreme lethargy, confusion and ataxia. HAPE appears as breathlessness even at rest, weakness and cough.