At one time or another you’ve probably had to force yourself to stay awake, maybe while driving or when you’re in a looong lecture. In these situations, you were exerting control over your sleep-wake cycles. Narcolepsy is a disorder in which the ability to regulate sleep-wake cycles is impaired, so the normal boundaries between sleeping and being awake are weak, leading to frequent lapses into sleep and the occurrence of elements of sleep while a person is awake.
In a deep part of the brain called the hypothalamus, there is a special group of neurons that help stabilize wakefulness and sleep. These neurons produce the neurotransmitters orexin A and B, also called hypocretin 1 and hypocretin 2, which connect to key sites in the brainstem and elsewhere that regulate wake and sleep states. Specifically, orexins have an excitatory effect, which helps stabilize wakefulness across the day, and sleep throughout the night.
In individuals with the classic form of narcolepsy, an autoimmune process kills off nearly all the orexin-producing neurons during adolescence, resulting in five key symptoms: daily sleepiness despite adequate sleep at night; episodes of muscle weakness known as cataplexy; an inability to move at the start or end of sleep; vivid hallucinations at the start or end of sleep; and fragmented sleep.
Sleepiness is usually the most challenging symptom. People with narcolepsy can doze off with little warning, usually when sitting down, like in a class or while working at a computer, but they generally don’t sleep more than healthy people in a given 24 hour period. Most individuals with narcolepsy find that a short, 15-minute nap substantially improves their alertness for a few hours, which suggests that the sleepiness of narcolepsy is caused by a problem with the brain circuits that normally promote full alertness, rather than poor quality or insufficient sleep. Normally when a healthy person goes to bed, they go through a sleep cycle lasting an hour or more before they reach rapid eye movement, or REM sleep, which is the stage of sleep characterized by dreaming. People with narcolepsy fall asleep very quickly, in as little as five minutes, and they often enter REM sleep in just a few minutes. This can result in very vivid dreams, even with brief naps.