Narcolepsy is a chronic neurological disorder that affects the brains ability to control sleep-wake cycles. People with narcolepsy may feel rested after waking up, but then feel very drowsy during the day. The exact cause of narcolepsy is not known, but it is believed to involve multiple factors that come together to cause problems in the brain and disturb REM sleep. However, researchers have found that people with type 1 narcolepsy have low levels of hypocretin, also called orexin, which is a chemical in the brain that helps control being awake and when you enter REM sleep. Hypocretin levels are low in people who experience cataplexy, which is a sudden loss of muscle tone. The loss of hypocretin-producing neurons in the hypothalamus is the primary cause of type 1 narcolepsy. Researchers believe that in individuals with narcolepsy, the bodys immune system selectively attacks the hypocretin-containing neurons. However, this does not explain all cases of narcolepsy, and the exact cause of the problem is often unclear. Some things that have been suggested as possible triggers of narcolepsy include hormonal changes, major psychological stress, and an infection, such as swine flu, or the medicine used to vaccinate against it (Pandemrix) .