Since Gelineau's initial description, the definition of narcolepsy has changed slightly to encompass these four symptoms: excessive sleepiness and/or uncontrolled episodes of overwhelming sleepiness (often called "sleep attacks"); hypnagogic hallucinations (realistic imagery occurring with the onset of sleep); sleep paralysis (inability to move just as one is awakening or going to sleep); and cataplexy (sudden, brief loss of muscle tone triggered by strong emotions).

It appears at the onset of sleep and acts as a transition stage into Stage 2: This stage can be associated with hypnagogic hallucinations (dreamlike images, sounds or sometimes smells before falling asleep or waking up), a loss of muscle tone, and a conscious awareness of the external environment.

Daytime sleepiness, sleep paralysis, and hypnagogic hallucinations also occur in people who do not have Narcolepsy, more frequently in people who are suffering from extreme lack of sleep.