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The cause of hypersomnia often remains unknown. Genetics appears to play a role in some types of hypersomnia, but how the condition is inherited is unknown.

Several different medical conditions and disorders also may cause patients to feel extremely drowsy. Typically, these conditions are not considered to be examples of hypersomnia. Instead, they are viewed as causes of hypersomnia symptoms. Several sleep disorders are associated with hypersomnia. They include:

• Narcolepsy. A neurological condition in which the brain is unable to properly regulate a person’s sleep-wake cycle. Although similar to hypersomnia, it can usually be distinguished by features that include a tendency to suddenly fall asleep and cataplexy (a loss of muscle control when experiencing strong emotions). Adolescents who experience recurrent bouts of hypersomnia are often later properly diagnosed as having narcolepsy (rather than primary hypersomnia).
  
  
• Breathing-related sleep disorders. Conditions in which abnormal breathing interferes with a person’s ability to sleep, leading to hypersomnia. Obstructive sleep apnea is the most common type of breathing-related sleep disorder.
  
  
• Movement disorders that occur during sleep. Patients with conditions that cause movement during sleep (e.g., restless leg syndrome, periodic limb movement disorder) may not sleep well, which can lead to hypersomnia.

Various medical problems also can cause hypersomnia, including head injuries, tumors or damage to the central nervous system. Medical conditions that can cause hypersomnia include depression, bipolar disorder, epilepsy, heart problems, hypercalcemia, hyperthyroidism, liver problems, lung problems, multiple sclerosis, obesity and brain infections (e.g., meningitis, encephalitis).

Other conditions that may trigger hypersomnia include the following:

• Autonomic nervous system dysfunction. The autonomic nervous system regulates physiologic processes in the body that are not under a person’s control, such as blood pressure. When this system is impaired, it can lead to hypersomnia.
  
  
• Chronic fatigue syndrome. Condition in which a patient experiences prolonged tiredness that is not relieved by rest. Hypersomnia often is associated with this condition.
  
  
• Drug or alcohol abuse. Use of various illegal and legal drugs and medications can cause hypersomnia. For example, patients who abuse sleep-aid drugs may experience chronic drowsiness.
  
  
• Use of certain medications. Use of some medications (e.g., tranquilizers) may cause hypersomnia. Generally, hypersomnia abates once a patient stops using these drugs.  

Young people may be more at risk of hypersomnia than older people. Hypersomnia most often occurs in people between the ages of 15 and 30 years old. Gender may be a risk factor for some types of hypersomnia. For example, the recurrent primary hypersomnia known as Kleine-Levin syndrome affects men more often than women.