Obstructive sleep apnea is when something blocks part or all of your upper airway while you sleep. Your diaphragm and chest muscles have to work harder to open your airway and pull air into your lungs. Your breath can become very shallow, or you may even stop breathing briefly. You usually start to breathe again with a loud gasp, snort, or body jerk. You may not sleep well, but you probably won’t know that it’s happening. The condition can also lower the flow of oxygen to your organs and cause uneven heart rhythms.