Hypersomnolence is excessive sleepiness. People with hypersomnolence, as opposed to fatigue, often fall asleep unintentionally. Once hypersomnolence has been confirmed, the first cause to be ruled out should be inadequate sleep time (chronic insufficient sleep). Most people require at least 7 hours of sleep per night and often closer to 8, men generally requiring less than women. Hypersomnia is fairly common, with nearly 5% of the population affected. The age groups most affected by hypersomnia are teenagers and young adults. As with most sleep disorders, hypersomnia is underreported because many people inaccurately believe that always feeling sleepy and taking naps are normal behavior.

There are perhaps 5 types of hypersomnia:

1. Post-traumatic hypersomnia may arise from a head injury or a traumatic incident involving the central nervous system. It is usually associated with related symptoms such as headaches, fatigue, memory impairment, as well as difficulty concentrating. Usually experienced immediately after the accident, some cases are delayed for more than a year. Duration of symptoms may last only a short time, can continue for months or years, or can remain indefinitely.
2. Recurrent hypersomnia. Kleine-Levin Syndrome, a disorder that causes individuals to sleep for extended periods of time followed by a normal period of alertness, is a type of recurrent hypersomnia. The length of the episode or the time period between episodes varies. It may or may not be accompanied by other symptoms such as overeating and hypersexuality.
3. Idiopathic (primary) hypersomnia. This category is for those cases for which a cause cannot be determined. This type of hypersomnia is similar to narcolepsy in that the individual is excessively sleepy, falls asleep at inappropriate times, frequently takes naps, and sleeps at night for greater than 10 hours. In some cases of idiopathic hypersomnia, cataplexy and hypnagogic hallucinations may be present and resemble the symptoms of narcolepsy. However, in idiopathic hypersomnia there are insufficient sleep onset REM periods to justify the diagnosis of narcolepsy. Idiopathic hypersomnia is a lifelong disorder with no tendency to remit spontaneously.
4. Normal hypersomnia. A "normal" hypersomniac is one who simply requires more than 10 hours sleep each day. There is a genetic predisposition to this type of hypersomnia. These individuals are typically called "long sleepers."
5. Psychiatric hypersonmia is when excessive daytime sleepiness (EDS) is judged to be related to a mental disorder. This is a challenging area because the EDS may also produce mental disorders (mood disorders) as a complication of hypersomnia. A sleep study can help determine which came first, the chicken or the egg. [Psychosomatic Medicine 2000, 62: pp.220-226]

• Sleep apnea (or sleep apnoea) syndrome
• Narcolepsy
• Primary CNS hypersomnolence
• Periodic movements of the legs during sleep (PMLS), also known as nocturnal myoclonus. A condition where the legs move every 40 seconds or so, producing brief arousals. Commonly seen in the first few weeks on nasal CPAP, but it is not understood why. PMLS is another cause of daytime sleepiness.
• rare forms of hypersomnolence such as post traumatic hypersomnolence, periodic hypersomnolence (Kleine-Levin Syndrome, idiopathic recurring stupor), or post-infectious hypersomnia.