Altitude sickness can result when you’ve moved to high elevations without having time to adjust. When you're at a mile (5,280 feet) or more above sea level, and especially when you're higher than 8,500 feet, you may feel symptoms of altitude sickness: shortness of breath, light-headedness, nausea, fatigue, headache, and insomnia. To help your body adjust, drink lots of water, avoid alcohol, and wait a day or two before attempting vigorous activity. If your symptoms are severe, last several days, or worsen, seek medical attention.