1. A condition characterized by abnormally long or frequent periods, or abnormal depth, of sleep: Susan's hypersomnia affected her daily life in that she was exceedingly tired and sleeping more and more during the day and not tending to her household chores.
2. Excessive or uncontrollable sleepiness: After spending days and nights with constant work in the emergency hospital, Dr. Smith suffered from hypersomnia, sleeping through his meal times at home."
3. A condition in which sleep periods are excessively long, but the person responds normally in the intervals; distinguished from somnolence: After flying from California to Germany, David was so tired he went directly to bed and slept 10 hours, only getting up to eat with his family.

After eating, his hypersomnia took over again, and so he went back to bed and got up later, after several hours.

4. Etymology: from Greek hyper, "over, very much" + Latin somnus, -somnia, "sleep".