Many people have narcolepsy for 10 to 15 years before the disorder is correctly diagnosed.  Symptoms often develop slowly and go unrecognized. The severity can vary widely, so that a medical disorder is not suspected and help not sought until symptoms become severe.

Physicians often misdiagnose another illness, or think the sleepiness is a side effect of a medication. Cataplectic attacks have also been misdiagnosed as epileptic seizures, though narcolepsy is not a form of epilepsy.

Narcolepsy can be confirmed by a test such as a polysomnography (PSG) and/or a Multiple Sleep Latency Test (MSLT), both of which are given in a sleep disorders clinic. People with narcolepsy are usually under the care of a neurologist or specialist sleep physician.