Primary hypersomnia

Hypersomnia is an excessive amount of sleepiness, resulting in an inability to stay awake.

Even though hypersomniacs sleep more than is considered necessary (or even healthy) at night, they may still need to take frequent naps during the day.

Diagnosis
A person is considered to have hypersomnia if he or she sleeps more than 10 hours per day on a regular basis for at least two weeks.

Causes
People who are overweight are more likely than others to suffer from hypersomnia, and this can often exacerbate their weight problems as their excessive sleep patterns mean they will use up less energy, making it even harder to lose weight.

Known causes include heredity and brain damage.

Another possible cause is an infection of mononucleosis, as several instances of idiopathic hypersomnia have been found to start up immediately after such an infection (Dr. Givan, MD, Riley Hospital).

Treatment
Hypersomnia can be treated with stimulants such as Modafinil.