The diagnosis of narcolepsy can be relatively straight forward if all of its symptoms are present.  The co-occurrence of all symptoms with sleepiness at disease onset is, however, the exception and not the rule.  Moreover, because hallucinations and sleep paralysis are not unique to narcolepsy (i.e., they can be caused by other non-narcolepsy related diseases of sleepiness and are often present even in the absence of a sleep disorder), and cataplexy can be absent or mild at disease onset, or difficult to differentiate from the norm (e.g., the colloquialisms “my knees buckled with fear” and “jaw dropped with surprise”), additional testing is usually performed.  Most typically a night-time sleep test (polysomnogram) is performed followed by multiple sleep latency testing (MSLT) the day after during which the patient is given an opportunity to fall asleep on 4 or 5 occasions at 2-hour intervals after waking up.  This testing performed at a sleep laboratory rules out other causes of sleepiness such as obstructive sleep apnea, and the MSLT in patients with narcolepsy usually demonstrates an average latency to sleep of < 5 minutes, and the intrusion of dream sleep into 2 or more of the daytime naps.  If the results do not paint a clear picture (e.g, narcolepsy is suggested in the absence of cataplexy, or the MSLT results are ambiguous) some specialized centers will perform a lumbar puncture to obtain cerebrospinal fluid (CSF) in order to measure hypocretin.  In subjects with narcolepsy lacking cataplexy, only 10-20% have hypocretin values in the CSF that are below the normal range, and therefore, they are likely to represent a unique subgroup of patients distinct from narcolepsy caused by hypocretin deficiency (“narcolepsy type 1” – is a proposed terminology from the upcoming 3rd edition of the International Classification of Sleep Disorders; ICDS-3) . A portion of these seem more closely related to patients with idiopathic hypersomnia based upon shared symptoms and biological markers (see separate section discussing Idiopathic Hypersomnia).