Hypersomnia, a condition where a person can sleep for 12-15 hours but still wake up feeling tired.
Hypersomnia, or excessive sleepiness, is a condition in which a person has trouble staying awake during the day. People who have hypersomnia can fall asleep at any time; for instance, at work or while they are driving. They may also have other sleep-related problems, including a lack of energy and trouble thinking clearly.
Source: Weird Facts