The essential features of sleepiness in narcolepsy is irresistible attacks of refreshing sleep that occur almost daily (at least 3x per week) over at least 3 months. Narcolepsy generally produces cataplexy, which most commonly presents as brief episodes (seconds to minutes) of sudden, bilateral loss of muscle tone precipitated by emotions, typically laughing and joking. Muscles affected may include those of the neck, jaw, arms, legs, or whole body, resulting in head bobbing, jaw dropping, or complete falls. Individuals are awake and aware during cataplexy.

Narcolepsy-cataplexy affects 0.02%–0.04% of the general population in most countries. Narcolepsy affects both genders, with possibly a slightly greater prevalence in males. In 90% of cases, the first symptom to manifest is sleepiness or increased sleep, followed by cataplexy (within 1 year in 50% of cases, within 3 years in 85%).

Onset is typically in children and adolescents/young adults but rarely in older adults. Two peaks of onset are generally seen, at ages 15–25 years and ages 30–35 years. Onset can be abrupt or progressive (over years). It is most severe when it occurs abruptly in children. Illustratively, sleep paralysis usually develops around puberty in children who have prepubertal onset. Since 2009, clinicians have observed greater rates of abrupt onset in young children who are obese and likely to experience premature puberty. In adolescents, onset is more difficult to pinpoint. Onset in adults is often unclear, with some individuals reporting having had excessive sleepiness since birth. Once the disorder has manifested, the course is persistent and lifelong.

Sleepiness, vivid dreaming, and excessive movements during REM sleep are early symptoms. Excessive sleep rapidly progressing to an inability to stay awake during the day is indicative of its progression. Within 6 months of onset, spontaneous grimaces or jaw-opening episodes with tongue thrusting (often a precursor of later developing cataplexy) is a common symptom in individuals with this disorder.  Exacerbation of symptoms suggest lack of compliance with medications or development of a concurrent sleep disorder, notably sleep apnea. Some medication treatments are helpful and can lead to the disappearance of cataplexy.

The specific symptoms in the DSM-5 requires presence of recurrent periods of an irrepressible need to sleep, lapsing into sleep, or napping occurring within the same day (3x per week over the past 3 months) (Criteria A) PLUS at least one of the following Criterion B symptoms:

  1. Cataplexy (i.e., brief episodes of sudden bilateral loss of muscle tone, most often associated with intense emotion)
  2. Hypocretin deficiency, as measured using cerebrospinal fluid (CSF)
    • Laboratory test results must reveal hypocretin-1 immunoreactivity values of less than or equal to one-third of values obtained in healthy subjects (or less than or equal to 110 pg/mL).
  3. Results of a formal sleep study (nocturnal sleep polysomnography) conducted by a medical professional showing abnormal rapid eye movement (REM) sleep latency (e.g., ≤ 15 minutes). This manifests as recurrent intrusions of elements of rapid eye movement (REM) sleep into the transition between sleep and wakefulness, as manifested by either hypnopompic or hypnagogic hallucinations or sleep paralysis at the beginning or end of sleep episodes

The severity of the disorder depends on the frequency of cataplexy or response to medication treatment. Mild narcolepsy indicates infrequent cataplexy (less than once per week), need for naps only once or twice per day, and less disturbed nocturnal sleep; Moderate indicates cataplexy once daily or every few days, disturbed nocturnal sleep, and need for multiple naps daily; and severe as drug-resistant cataplexy with multiple attacks daily, nearly constant sleepiness, and disturbed nocturnal sleep (i.e., movements, insomnia, and vivid dreaming).