Narcolepsy is a neurologic disorder of rapid eye movement (REM) sleep that can be either associated with cataplexy or without cataplexy. Narcolepsy affects men and women equally with an incidence that varies among ethnic groups. It is more common among the Japanese (1:600) and less common among Israeli Jews (1:500,000). On average, it is estimated to have an incidence of 1:2,000 people. Typically, the onset of symptoms is insidious, and the diagnosis is often delayed by several years. The peak age of presentation is 15 years old with a second peak at around 35 years of age. First-degree relatives of patients with narcolepsy have a 1 % risk of developing this condition [2].

The etiology of narcolepsy is thought to be mainly an autoimmune response against hypocretin-producing neurons in the hypothalamus. Hypocretin neurons serve as a stabilizer of sleep and regulate REM sleep through a physiologic on/off switch. Since this switch is not working properly, patients with narcolepsy not only experience sleepiness during the day but also experience disturbed sleep at night with multiple brief nocturnal awakenings [3]. When patients with narcolepsy fall asleep, they usually start dreaming within 15 min of sleep onset (sleep onset REM), a neurophysiologic marker that is assessed during the multiple sleep latency test (MSLT) and is useful in diagnosis of the condition. Hypocretin deficiency in cerebrospinal fluid with a level ≤110 pg/mL is diagnostic of narcolepsy.

Excessive daytime sleepiness (EDS) is usually the presenting symptom in patients with narcolepsy and is found in 100 % of narcoleptics. EDS is usually severe with sudden “sleep attacks” throughout the day. Naps are usually refreshing. Diagnostic criteria for narcolepsy require EDS to be present for at least 3 months. There are several scales and questionnaires that assess the degree of sleepiness; these tools are discussed elsewhere.

Cataplexy, the pathognomonic symptom of narcolepsy, is defined as a sudden decrease or loss of muscle tone triggered by emotions such as laughter, surprise, or anger. The loss of muscle tone can involve many muscles leading to falling to the ground or can be more subtle with loss of facial expression or weakness of the neck. It is found in 40 % of patients with narcolepsy. Cataplexy has been reported to be the presenting symptom of narcolepsy but usually develops after the presentation of EDS, and it can occur many years after the diagnosis of narcolepsy has been made. Cataplexy may be confirmed by checking deep tendon reflexes (DTRs) during the cataplexy attack. DTRs should be diminished even in unaffected muscles [4].

The next symptoms found in narcolepsy, hypnagogic hallucinations and sleep paralysis, may be seen in normal people especially with sleep deprivation and are thus not pathognomonic for narcolepsy. Hypnagogic hallucinations are a manifestation of REM dreaming intrusion into wakefulness and usually occur during the transition from wake to sleep. When these visions occur during the transition from sleep into wake, they are called hypnopompic hallucinations. Patients with tumors or lesions in the areas that affect REM control may develop these hallucinations as well.

Neural mechanisms prevent us from acting out our dreams during REM sleep. This normal “paralysis” during REM known as REM atonia is altered in patients with narcolepsy. Sleep paralysis is the intrusion of the normal atonia of REM sleep into wakefulness. The patient experiences inability to move with preserved awareness of surroundings. Sleep paralysis typically occurs during sleep–wake transitions or arousals. An increased number of leg movements at night and dream enactment behavior have been reported in patients with narcolepsy, but even in the absence of these movements, REM sleep without atonia has been observed in narcolepsy [5].