Obstructive sleep apnea (OSA) occurs when the tissue in the back of the throat collapses during sleep. This keeps air from getting into the lungs. This is very common, because the muscles inside the throat relax as you sleep. Gravity then causes the tongue to fall back and block the airway. It can happen a few times a night or several hundred times per night. These pauses in breathing briefly wake you up and disturb your sleep. This can cause you to be very tired the next day. People who are overweight are at a higher risk of having sleep apnea.
If you are having daytime tiredness and feel you are not sleeping well for any reason, talk with your doctor about your symptoms and possible causes. In some cases, a sleep study may be recommended.
— Source: UCLA Sleep Disorders Center