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There are many causes of excessive sleepiness, and sometimes an underlying sleep disorder is responsible. Most sleep disorders disrupt a person's slumber—usually by shortening the length of time asleep or reducing the quality of sleep—making the person drowsy during the day as a result.
One fairly common reason for excessive sleepiness is obstructive sleep apnea. People with Obstructive Sleep Apnea (OSA) struggle to breathe during the night and wake up many times per hour as a result. These wake ups continually interrupt the natural cycles of sleep, with all its lighter and deeper stages. As a result, the person doesn't have sufficient and healthy sleep and feels drowsy the next day. Many people with OSA don't realize they have it, because they wake up so briefly they believe they have slept continuously through the night (in fact they may have woken up a hundred times or more). If you are excessively sleepy during the day and especially if you snore, have high blood pressure, or are overweight (factors associated with OSA), it's a good idea to have your doctor investigate the possibility of this sleep disorder.