Narcolepsy with cataplexy is caused by a lack of the chemical hypocretin in the brain. Hypocretin is an important chemical for regulating wakefulness and rapid eye movement (REM) sleep. Narcolepsy without cataplexy includes most of the same symptoms, but its cause is unknown. Injuries to the hypothalamus and brain stem, tumors, and stroke are under investigation.
The current understanding of narcolepsy is that it begins with an underlying genetic predisposition; a person is born with certain genes that put them at greater risk for developing narcolepsy. In childhood or the teen years, an event like an infection may trigger the onset of narcolepsy. Instead of the immune system simply attacking the infection, it becomes confused and attacks the specialized cells in the brain that produce hypocretin. The loss of hypocretin-producing cells leads to the symptoms of narcolepsy. Because the immune system is believed to be responsible for the loss of these cells, narcolepsy is considered an autoimmune disease. One of the triggering events linked to narcolepsy is an infection of the H1N1 influenza virus.
The loss of hypocretin in the brain makes it hard for a person to stay awake. It also allows REM sleep to occur in moments when it normally does not. A person with narcolepsy can enter REM sleep directly from a waking state, rather than going through the normal progression from awake, through deep sleep, and eventually into REM sleep.