Sleep Apnea is when you stop breathing while you are sleeping. Your body needs to breathe in oxygen to stay alive, and when that doesn't happen it can make you very sick. When you have sleep apnea, the muscles that control your breathing relax too much or don't work correctly, which causes you to take very shallow breaths or even stop breathing for a few seconds at a time, many times during the night. This can make it hard for your body to get enough oxygen, which can make you tired during the day. People with sleep apnea might snore a lot or wake up a lot during the night.