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.
Insomnia is another common cause of excessive sleepiness, in which a person has difficulty falling asleep or staying asleep for the 7-9 hours the body requires. People with insomnia are aware of their sleep struggles, but often don't consult with a doctor to receive proper treatment.
Another less common sleep disorder that causes excessive sleepiness is narcolepsy. A person with narcolepsy unwillingly and briefly falls asleep during the day—while talking, eating a meal, or even driving. In addition to feeling drowsy during the day, people with narcolepsy often have disturbed sleep at night, which compounds the problem of daytime sleepiness. Most cases of narcolepsy are caused by the dysfunction of a certain area in the brain that promotes wakefulness.
There are behavioral, psychological, and medical treatment options for sleep disorders. If you feel excessively sleepy, it's important to talk to your doctor—if a sleep disorder is the root, you can get started working on a strategy to treat it, improve your night's sleep, and feel alert and productive the next day.