Narcolepsy type I affects 25 to 50 per 100,000 individuals and narcolepsy type II affects 20 to 34 per 100,000 individuals. Males are affected more commonly than females.

Some of the human nerve cells contain a chemical called hypocretin. Hypocretin regulates various activities including wakefulness and arousal. During wakefulness, Hypocretin stimulates the arousal system that inhibits REM sleep, one of the important stages of sleep characterized by rapid eye movement (REM). Narcolepsy type I occurs due to the loss of hypocretin-containing nerve cells. In individuals with narcolepsy type I, REM sleep can no longer be inhibited properly during wakefulness, resulting in sudden-onset attacks of sleepiness. Hypocretin is also thought to increase muscle tone. Muscle tone is the consistent basal muscle tension that is essential to maintain the body in a certain position. When hypocretin is lost, muscle tone is also reduced, leading to cataplexy or the severe weakness of muscles. The cause of narcolepsy type II is not well-understood but it might involve the less severe loss of hypocretin-containing nerve cells or other mechanisms that affect the function of hypocretin.

The reason for the loss of hypocretin in narcolepsy type I is not clear, but some evidence suggests an autoimmune process where the immune system attacks narcolepsy-containing nerve cells. This autoimmunity is likely caused by a combination of genetic and environmental factors. Research suggests that previous infection by certain bacteria (Streptococcus) might act as an environmental factor that can trigger narcolepsy.

There is also evidence suggesting that genetic predisposition increases the risk of narcolepsy. Human leukocytes antigen (HLA)  are genes involved in regulating the function of the immune system. Subtypes of this gene, DR2 (DRB1*1501) and DQ (DQB1*0602), are closely associated with narcolepsy type I and less so with narcolepsy type II. However, these genes are also present in about a quarter of the general population as well.

Narcolepsy is associated with five classical symptoms which are excessive daytime sleepiness, cataplexy, hypnagogic hallucinations, sleep paralysis, and disrupted night sleep. The symptoms often begin either during mid-teens or mid-thirties.

Excessive daytime sleepiness means that the affected individual is unable to stay alert during waking hours and experiences general fatigue, lack of energy, and drowsiness. Excessive daytime sleepiness occurs despite prolonged night-time sleep. It is often experienced with sleep attacks which are episodes of overwhelming and strong sleepiness that often can’t be overcome, causing the individual to fall asleep for a short period of time. These sleep attacks are recurrent and last less than ten minutes and may occur at any time, regardless of the situation. They may occur as the individual is watching television, walking, or even driving. Individuals affected by narcolepsy often feel significantly more alert right after a short nap.

Excessive daytime sleepiness and sleep attacks are the most common symptoms. Cataplexy isn’t present in everyone but when it is present with excessive daytime sleepiness, it is a strong indicator of narcolepsy. Cataplexy is a sudden, significant muscle weakness that occurs when the affected individual experiences strong emotions such as anger embarrassment, surprise, and particularly laughter. The individual remains completely conscious during these episodes which last a few seconds to minutes but may be unable to move. During a cataplectic attack, the individual may also find it difficult to speak.

Cataplexy presents slightly differently in children. Cataplexy in children may involve persistent muscle weakness (hypotonia). Affected children may fall to the ground for a few seconds or show a typical “cataplectic facies.” Cataplectic facies describes distinct facial movement including eyebrows raising, tongue moving, eyelids drooping, jaw dropping, and having a facial expression that mimics disgust or pain. When present, cataplexy usually develops a few weeks after the onset of excessive daytime sleepiness. In some instances, it may occur even before excessive daytime sleepiness.

Hypnagogic hallucinations are experiences similar to dreaming that occur as the affected individual is falling asleep. These experiences are vivid pictures and sounds such as the sound of a phone ringing or seeing a person in the room who isn’t there. While these hallucinations may be pleasant, they are usually frightening. Hypnagogic hallucinations may occur with sleep paralysis.

Similar to hypnagogic hallucinations, sleep paralysis occurs as the individual is waking up. When experiencing sleep paralysis, the individual is briefly unable to speak, move or even open their eyes but can hear everything in their surrounding. Some may experience a feeling of heaviness on their chest. Affected individuals retain the ability to go to normal movement and function after a brief period.

Narcolepsy may also be present with disrupted or fragmented sleep during the night. Affected individuals may regularly wake up during the night and have difficulty falling back asleep.

Other symptoms may include impaired concentration and memory, blurry vision, depression and anxiety, and automatic behavior. Automatic behavior describes performing certain acts without conscious control. The individual often has no memory of performing that act. Prolonged sleep, weight gain, and aggressive behavior may be seen in children.

The diagnosis of narcolepsy is made based on history, the presence of excessive daytime sleepiness, and the measurement of hypocretin levels in cerebrospinal fluid. Cerebrospinal fluid is a protective fluid surrounding the brain and the spinal cord that contains many of the substances present in the brain or the spinal cord.

Excessive daytime sleepiness can be determined using certain scales and questionnaires or neurological tests. These scales are often questionnaires in which the individuals rate their daytime sleepiness, the likelihood of falling asleep during different situations, etc. It is also important to ensure that the individual is getting enough sleep on a regular basis to accurately evaluate the presence of excessive daytime sleepiness.

Low cerebrospinal fluid hypocretin levels suggest narcolepsy type I.

An important step in diagnosing narcolepsy is ensuring that the individual is getting enough sleep. This may be done using a sleep log filled out by the individual or actigraphy. An actigraph is a monitoring device that can be worn like a watch and measures sleep indicators and muscle movement.

If the patient is getting enough sleep, polysomnography (PSG) is done. PSG is usually done overnight and measures many physiological parameters such as heart rate, muscle movement, breathing pattern, eye movement, brain activity, etc. PSG can rule out other sleep disorders such as difficulty breathing at night (sleep apnea).

Multiple Sleep Latency Test (MSLT) is another test that is performed during the day, often following a PSG. It measures excessive daytime sleepiness. During MSLT, the individual is given five schedule napping opportunities, each lasting twenty minutes and two hours apart.  In a normal sleep cycle, individuals enter REM sleep after about 60-90 minutes of sleeping but individuals affected by narcolepsy enter this stage of sleep much more rapidly. An MSLT can indicate which stage of sleep the individual experiences at every moment. Individuals affected by narcolepsy are expected to enter REM sleep in less than fifteen minutes.

Furthermore, a lumbar puncture may be performed to take a sample from the cerebrospinal fluid to measure hypocretin levels. A lumbar puncture is a procedure in which a hollow needle is used to remove a small amount of cerebrospinal fluid from the space around the spinal cord.

The treatment of narcolepsy includes lifestyle changes as well as medications to alleviate the symptoms. Treatment focuses on reducing excessive daytime sleepiness, cataplexy, hypnagogic hallucinations, sleep paralysis, and disruption of night-time sleep. To treat narcolepsy, medications are often required. These medications may be wakefulness-promoting agents and central nervous system stimulants. In some cases, some antidepressants may also be effective. In case of excessive daytime sleepiness with cataplexy, a central nervous system sedative may be used.

Lifestyle changes can also alleviate excessive daytime sleepiness. A few schedule naps throughout the day can improve alertness. Adequate and regular night-time sleep, avoiding coffee or alcohol before sleeping, daily exercise, and avoiding large meals before bed are among helpful strategies to reduce excessive daytime sleepiness.

Narcolepsy does not affect life expectancy but it is a life-long disorder that can negatively impact the quality of life. While treatment can alleviate the symptoms, normal alertness may not always be achieved. The severity of symptoms varies among individuals and some may require accommodations at work or school or take precautions when driving.

Slowik JM, Yow AG. Narcolepsy. [Updated 2019 May 5]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459236/

Akintomide GS, Rickards H. Narcolepsy: a review. Neuropsychiatr Dis Treat. 2011;7:507–518. doi:10.2147/NDT.S23624

Abad VC, Guilleminault C. New developments in the management of narcolepsy. Nat Sci Sleep. 2017;9:39–57. Published 2017 Mar 3. doi:10.2147/NSS.S103467

Ahmed I, Thorpy M. Clinical Features, Diagnosis and Treatment of Narcolepsy. Clinics in Chest Medicine. 2019; 31(2): 371-381.

Postiglione E, Antelmi E, Fabio P, Lecendreux M, Dauvilliers Y, Plazzi G.The clinical spectrum of childhood narcolepsy. Sleep Disorder Reviews. 2018;38:70-85. Doi: http://dx.doi.org/10.1016/j.smrv.2017.04.003

National Institute of Neurological Disorders. Narcolepsy Fact Sheet. Available from https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Fact-Sheets/Narcolepsy-Fact-Sheet