In 1999, Louis Ptáček's and Ying-Hui Fu's research group at the University of California, San Francisco reported findings of a human circadian rhythm disorder showing a familial tendency. The disorder was characterized by a lifelong pattern of sleep onset around 7:30 p.m. and offset around 4:30 a.m. Among three lineages, 29 people were identified as affected with this familial advanced sleep-phase disorder (FASPD), and 46 were considered unaffected. The pedigrees demonstrated FASPD to be a highly penetrant, autosomal dominant trait.

Two years after reporting the finding of FASPD, Ptáček's and Fu's groups published results of genetic sequencing analysis on a family with FASPD. They genetically mapped the FASPD locus to chromosome 2q where very little human genome sequence was then available. Thus, they identified and sequenced all the genes in the critical interval. One of these was Period2 (Per2). Sequencing of the hPer2 gene revealed a serine-to-glycine point mutation in the CKI binding domain of the hPER2 protein that resulted in hypophosphorylation of Per2 in vitro.

In 2005, Fu's and Ptáček's labs reported discovery of a different mutation causing FASPD. This time, CKIδ was implicated, demonstrating an A-to-G missense mutation that resulted in a threonine-to-alanine alteration in the protein. The evidence for both of these reported causes of FASPD is strengthened by the absence of said mutations in all tested control subjects and by demonstration of functional consequences of the respective mutations in vitro. Fruit flies and mice engineered to carry the human mutation also demonstrated abnormal circadian phenotypes although the mutant flies had a long circadian period while the mutant mice had a shorter period. The differences between flies and mammals that account for this difference are not known. Most recently, Ptáček and Fu reported additional studies of the human Per2 S662G mutation and generation of mice carrying the human mutation. These mice had a circadian period almost 2 hours shorter than wild-type animals. Genetic dosage studies of CKIδ on the Per2 S662G mutation revealed that CKIδ is having opposite effects on Per2 levels depending on the sites on Per2 that CKIδ is phosphorylating.

Advanced sleep phase disorder (ASPD), also known as the advanced sleep-phase type (ASPT) of circadian rhythm sleep disorder or advanced sleep phase syndrome (ASPS), is a condition in which patients feel very sleepy and go to bed early in the evening (e.g. 6:00–8:00 p.m.) and wake up very early in the morning (e.g. around 3:00 a.m.).

There are an estimated 140,000 people with N24 – both sighted and blind – in the European Union, a total prevalence of approximately 3 per 10,000, or 0.03%. It is unknown how many individuals with this disorder do not seek medical attention, so incidence may be higher. The European portal for rare diseases, Orphanet, lists Non-24 as a rare disease by their definition: fewer than 1 affected person for every 2000 population. The US National Organization for Rare Disorders (NORD) lists Non-24 as a rare disease by its definition.

The direct cause and pathophysiological basis of RMD is still unknown and can occur in children and adults of perfect or non-perfect health. Rare cases of adult RMD have developed due to head trauma, stress, and herpes encephalitis. Familial cases have been reported suggesting there may be some genetic aspect to the disorder; however, to date, this explanation has not been directly tested. As familial incidence rate is still relatively low, it is believed that behavioral aspects may play a larger role in RMD than family history and genetics. Many sufferers report no family history of the disorder. Another theory suggests that RMD is a learned, self-stimulating behavior to alleviate tension and induce relaxation, similar to tic movements.

An alternative theory suggests that the rhythmic movements help develop the vestibular system in young children, which can partially explain the high prevalence of RMD in infants. It has been seen that children who have underdeveloped vestibular systems benefit from performing RMD-like movements which stimulate the vestibular system

As of 2005, there had been fewer than 100 cases of sighted people with non-24 reported in the scientific literature.

Sleep-related movements are commonly seen in children, especially infants. However, the majority of these movements stop as the child ages. Some 66% of infants of 9-months show RMD-like symptoms compared to only 8% of 4 year olds. The disorder is closely associated to mental retardation or other psychiatric disorders like Autism. More recent studies have shown there is a strong link between prolonged RMD and ADHD

DSPD is genetically linked to attention deficit hyperactivity disorder by findings of polymorphism in genes in common between those apparently involved in ADHD and those involved in the circadian rhythm and a high proportion of DSPD among those with ADHD.

Many studies indicate the effect of a "fight or flight" response on the body that happens with each apneic event is what increases health risks and consequences in OSA. The fight or flight response causes many hormonal changes in the body; those changes, coupled with the low oxygen saturation level of the blood, cause damage to the body over time.

Without treatment, the sleep deprivation and lack of oxygen caused by sleep apnea increases health risks such as cardiovascular disease, aortic disease (e.g. aortic aneurysm), high blood pressure, stroke, diabetes, clinical depression, weight gain and obesity.

The most serious consequence of untreated OSA is to the heart. Persons with sleep apnea have a 30% higher risk of heart attack or death than those unaffected. In severe and prolonged cases, increased in pulmonary pressures are transmitted to the right side of the heart. This can result in a severe form of congestive heart failure known as "cor pulmonale". Dyastolic function of the heart also becomes affected. One prospective study showed patients with OSA, compared with healthy controls, initially had statistically significant increases in vascular endothelial growth factor (P=.003) and significantly lower levels of nitrite-nitrate (P=.008), which might be pathogenic factors in the cardiovascular complications of OSA. These factors reversed to normal levels after 12 weeks of treatment by CPAP, but further long-term trials are needed to assess the impact of this therapy.

Elevated arterial pressure (i.e., hypertension) can be a consequence of OSA syndrome. When hypertension is caused by OSA, it is distinctive in that, unlike most cases (so-called essential hypertension), the readings do "not" drop significantly when the individual is sleeping (non-dipper) or even increase (inverted dipper).

Idiopathic hypersomnia is a lifelong disorder (with only rare spontaneous remissions) whose symptoms typically begin in adolescence or young adulthood. It is initially progressive, but may stabilize, and its main consequences are professional and social.

Idiopathic hypersomnia profoundly affects work, education, and quality of life. Patients are often too sleepy to work or attend school regularly, and they are predisposed "to develop serious performance decrements in multiple areas of function as well as to potentially life-threatening domestic, work-related and driving accidents." Furthermore, these risks are higher for idiopathic hypersomnia patients than for those with sleep apnea or severe insomnia. In fact, "the most severe cases of daytime somnolence are found in patients affected by narcolepsy or idiopathic hypersomnia." And idiopathic hypersomnia is often as, if not more, disabling than narcolepsy; surprisingly, excessive daytime sleepiness is even more handicapping than the cataplectic attacks of narcolepsy.

Due to the consequences of their profound EDS, both idiopathic hypersomnia and narcolepsy can often result in unemployment. Several studies have shown a high rate of unemployment in narcoleptics (from 30-59%), which was felt to be related to the severe symptoms of their illness.

The condition is thought to be under-reported in the medical literature. A study of 27 cases conducted by Timothy C Hain in 1999 noted all but one patient to be female. The average age in this series was 49 years. This apparent gender disparity, however, may be due in part to the fact that the questionnaire which formed the basis of the study was circulated in a publication with a predominantly female reader base.

Subsequent studies have produced conflicting results with regard to the gender distribution of MdDS. The trends in Hain's report have recently been supported by the MdDS Balance Disorder Foundation, in a study of over 100 individuals diagnosed with MdDS. The female:male ratio was approximately 9:1; the average age of onset was 43–45 years. However, another recent study found that 44% of subjects who had experienced MdDS for 2 years or more were male, suggesting a more even distribution.

It has been shown to occur in excursions of as little as 30 minutes though it has been unclear how long it takes for symptoms to occur. The most commonly reported inciting event was a prolonged ocean cruise (~45%); however, shorter boating excursions (~22%), aircraft travel (~15%), and automobile travel (~8%) have all been described.

Mal de Débarquement syndrome has been noted as far back to the times of Erasmus Darwin in 1796, and Irwin J A (1881) "The pathology of seasickness".

Cases of MdDS have been reported in children as young as eight and in both genders. Men may have a more difficult time obtaining a diagnosis due to the disparity of women reported. When sailors and soldiers returned from World War II, the syndrome was reported at a higher rate in males.

Males with pathogenic "MECP2" mutations usually die within the first 2 years from severe encephalopathy, unless they have an extra X chromosome (often described as Klinefelter syndrome), or have somatic mosaicism.

Male fetuses with the disorder rarely survive to term. Because the disease-causing gene is located on the X chromosome, a female born with an MECP2 mutation on her X chromosome has another X chromosome with an ostensibly normal copy of the same gene, while a male with the mutation on his X chromosome has no other X chromosome, only a Y chromosome; thus, he has no normal gene. Without a normal gene to provide normal proteins in addition to the abnormal proteins caused by a MECP2 mutation, the XY karyotype male fetus is unable to slow the development of the disease, hence the failure of many male fetuses with a MECP2 mutation to survive to term.

Females with a MECP2 mutation, however, have a non-mutant chromosome that provides them enough normal protein to survive longer. Research shows that males with Rett syndrome may result from Klinefelter's syndrome, in which the male has an XXY karyotype. Thus, a non-mutant "MECP2" gene is necessary for a Rett's-affected embryo to survive in most cases, and the embryo, male or female, must have another X chromosome.

There have, however, been several cases of 46,XY karyotype males with a MECP2 mutation (associated with classical Rett syndrome in females) carried to term, who were affected by neonatal encephalopathy and died before 2 years of age. The incidence of Rett syndrome in males is unknown, partly owing to the low survival of male fetuses with the Rett syndrome-associated MECP2 mutations, and partly to differences between signs caused by MECP2 mutations and those caused by Rett's.

Females can live up to 40 years or more. Laboratory studies on Rett syndrome may show abnormalities such as:

- EEG abnormalities from 2 years of age

- atypical brain glycolipids

- elevated CSF levels of "beta"-endorphin and glutamate

- reduction of substance P

- decreased levels of CSF nerve growth factors

A high proportion of deaths are abrupt, but most have no identifiable cause; in some instances death is the result most likely of:

- spontaneous brainstem dysfunction

- cardiac arrest, likely due to long QT syndrome, ventricular tachycardia or other arrhythmias

- seizures

- gastric perforation

In one case, a patient was diagnosed with both Morvan's syndrome and pulmonary hyalinizing granulomas (PHG). PHG are rare fibrosing lesions of the lung, which have central whorled deposits of lamellar collagen. How these two diseases relate to one another is still unclear.

Thymoma, prostate adenoma, and in situ carcinoma of the sigmoid colon have also been found in patients with Morvan’s Syndrome.

There have been many studies suggesting health risks associated with shift work. For example, a 2007 study led by the IARC (International Agency for Research on Cancer) showed that shiftwork has been associated with cancer. Other studies have reported that night workers have an increased incidence of heart disease, digestive disorders and menstrual irregularities. Because a formal diagnosis of SWSD was not typically made in these studies, it remains unclear whether the reported risks apply to the subset of shiftworkers who qualify for a diagnosis of SWSD or apply to all shiftworkers.

OSA accompanied by daytime sleepiness is estimated to affect 3% to 7% of men and 2% to 5% of women, and the disease is common in both developed and developing countries. It is most commonly diagnosed in middle-aged males.

If studied carefully in a sleep lab by polysomnography (formal "sleep study"), it is believed that approximately 1 in 5 American adults would have at least mild OSA.

Antibodies against voltage-gated potassium channels (VGKC), which are detectable in about 40% of patients with acquired neuromytonia, have been implicated in Morvan’s pathophysiology. Raised serum levels of antibodies to VGKCs have been reported in three patients with Morvan’s Syndrome. Binding of serum from a patient with Morvan’s Syndrome to the hippocampus in a similar pattern of antibodies to known VGKC suggest that these antibodies can also cause CNS dysfunction. Additional antibodies against neuromuscular junction channels and receptors have also been described. Experimental evidence exists that these anti-VGKC antibodies cause nerve hyperexcitability by suppression of voltage gated K+ outward currents, whereas other, yet undefined humoral factors have been implicated in anti-VGKC antibody negative neuromyotonia. It is believed that antibodies to the Shaker-type K+ channels (the Kv1 family) are the type of potassium channel most strongly associated with acquired neuromyotonia and Morvan’s Syndrome.

Whether VGKC antibodies play a pathogenic role in the encephalopathy as they do in the peripheral nervous system is as yet unclear. It has been suggested that the VGKC antibodies may cross the blood–brain barrier and act centrally, binding predominantly to thalamic and striatal neurons causing encephalopathic and autonomic features.

The most comprehensive assessment so far has estimated RBD prevalence to be about 0.5% in individuals aged 15 to 100. It is far more common in males: most studies report that only about a tenth of sufferers are female. This may partially be due to a referral bias, as violent activity carried out by men is more likely to result in harm and injury and is more likely to be reported than injury to male bed partners by women, or it may reflect a true difference in prevalence as a result of genetic or androgenic factors. The mean age of onset is estimated to be about 60 years.

Various conditions are very similar to RBD in that sufferers exhibit excessive sleep movement and potentially violent behavior. Such disorders include sleepwalking and sleep terrors, which are associated with other stages of sleep, nocturnal seizures and obstructive sleep apnea which can induce arousals from REM sleep associated with complex behaviors. Because of the similarities between the conditions, polysomnography plays an important role in confirming RBD diagnosis.

It is now apparent that RBD appears in association with a variety of different conditions. Narcolepsy has been reported as a related disorder. Both RBD and narcolepsy involve dissociation of sleep states probably arising from a disruption of sleep control mechanisms. RBD has also been reported following cerebrovascular accident and neurinoma (tumor), indicating that damage to the brain stem area may precipitate RBD. RBD is usually chronic. However, it may be acute and sudden in onset if associated with drug treatment or withdrawal (particularly with alcohol withdrawal). 60% of RBD is idiopathic. This includes RBD that is found in association with conditions such as Parkinson's disease and dementia with Lewy bodies, where it is often seen to precede the onset of neurodegenerative disease. Monoamine oxidase inhibitors, tricyclic antidepressants, Selective serotonin reuptake inhibitors, and noradrenergic antagonists can induce or aggravate RBD symptoms and should be avoided in patients with RBD.

Sexsomnia affects individuals of all age groups and backgrounds but present as an increased risk for individuals who possess the following:

- coexisting sleep disorders

- sleep disruption secondary to obstructive sleep apnea

- sleep related epilepsy

- certain medications

Behaviors of pelvic thrusting, sexual arousal, and orgasms are often attributed to sleep related epilepsy disorder. In some cases, physical contact with a partner in bed acted as a trigger to initiate sexsomia behaviors.

Medications, such as the commonly prescribed treatment for insomnia, Ambien, have been shown to induce symptoms commonly associated with sexsomnia.

Like sleep-related eating disorders, sexsomnia presents more commonly in adults than children. However, these individuals usually have a history of parasomnias that began during childhood.

Persons with obsessive-compulsive disorder are also diagnosed with DSPD at a much higher rate than the general public.

Panayiotopoulos syndrome is probably genetically determined, though conventional genetic influences may be less important than other mechanisms. Usually, there is no family history of similar seizures, although siblings with Panayiotopoulos syndrome or Panayiotopoulos syndrome and rolandic epilepsy or, less common, Panayiotopoulos syndrome and idiopathic childhood occipital epilepsy of Gastaut have been reported. There is a high prevalence of febrile seizures (about 17%).

SCN1A mutations have been reported in a child and in 2 siblings with relatively early onset of seizures, prolonged time over which many seizures have occurred, and strong association of seizures with febrile precipitants even after the age of 5 years. However, no such mutations were found in another couple of siblings and many other cases with typical Panayiotopoulos syndrome. These data indicate that SCN1A mutations when found contribute to a more severe clinical phenotype of Panayiotopoulos syndrome.

Symptoms of sexsomnia can be caused by or be associated with:

- stress factors

- sleep deprivation

- Consumption of alcohol or other drugs

- Pre-existing parasomnia behaviors

Sleep deprivation is known to have negative effects on the brain and behavior. Extended periods of sleep deprivation often results in the malfunctioning of neurons, directly effecting an individual's behavior. While muscles are able to regenerate even in the absence of sleep, neurons are incapable of this ability. Specific stages of sleep are responsible for the regeneration of neurons while others are responsible for the generation of new synaptic connections, the formation of new memories, etc.

Zolpidem, the widely known sedative Ambien, is used as common treatment for insomnia and has been seen to result in sexsomnia as an adverse effect.

Sexsomnia can also be triggered by physical contact initiated by a partner, or an individual sharing the same bed.

When infants have a lower birth weight or younger gestational age, there is a greater risk of infantile apnea. With the advancement of neonatal intensive care units and the greater technology available, there are more successful premature births compared to the past. With the greater number of premature infants being born, there is also a greater number of children with infantile apnea. Approximately 85 percent of infants born with a weight less than experience infantile apnea within the first month after birth. This risk decreases to 25 percent for infants weighing less than . Studies have found that almost 2% of the pediatric population experience obstructive sleep apnea.

Typically, the symptoms of idiopathic hypersomnia begin in adolescence or young adulthood, although they can begin at a later age. After onset, hypersomnia often worsens over several years, but it is often stable by the time of diagnosis and appears to be a lifelong condition. Spontaneous remission is only seen in 10-15% of patients.

According to the limited epidemiological data that exists, IH "has more of a female preponderance (1.8/1)." Family cases are frequent, in a range from 25% to 66% without any clear mode of inheritance."

Idiopathic hypersomnia has long been considered a rare disease, believed to be 10 times less frequent than narcolepsy. The prevalence of narcolepsy (with cataplexy) is estimated between 1/3,300 and 1/5,000. Although the true prevalence of idiopathic hypersomnia is unknown, it is estimated at 1/10,000 – 1/25,000 for the long sleep form and 1/11,000 to 1/100,000 without long sleep. A more precise estimate "is complicated by a lack of clear biologic markers" and a lack of "unambiguous diagnostic criteria."

Because idiopathic hypersomnia has been considered a rare disease, it has not received enough attention from authorities and researchers. "Patients are rare, researchers and scientists involved in the field are few and research findings are therefore scarce." "In Europe and in North America there is now a public health concern about helping patients and families affected by these rare diseases. Due to the complexity of the disease, they often experience difficulties to be diagnosed and often face social and professional consequences." (see Prognosis)

The exact prevalence of obesity hypoventilation syndrome is unknown, and it is thought that many people with symptoms of OHS have not been diagnosed. About a third of all people with morbid obesity (a body mass index exceeding 40 kg/m) have elevated carbon dioxide levels in the blood.

When examining groups of people with obstructive sleep apnea, researchers have found that 10–20% of them meet the criteria for OHS as well. The risk of OHS is much higher in those with more severe obesity, i.e. a body mass index (BMI) of 40 kg/m or higher. It is twice as common in men compared to women. The average age at diagnosis is 52. American Black people are more likely to be obese than American whites, and are therefore more likely to develop OHS, but obese Asians are more likely than people of other ethnicities to have OHS at a lower BMI as a result of physical characteristics.

It is anticipated that rates of OHS will rise as the prevalence of obesity rises. This may also explain why OHS is more commonly reported in the United States, where obesity is more common than in other countries.

In the United States, it is estimated that this condition afflicts as many as 200,000 Americans, but fewer than 50,000 are diagnosed. It is as widespread as Parkinson's disease or multiple sclerosis and more prevalent than cystic fibrosis, but it is less well known. Narcolepsy is often mistaken for depression, epilepsy, or the side effects of medications. It can also be mistaken for poor sleeping habits, recreational drug use, or laziness. Narcolepsy can occur in both men and women at any age, although its symptoms are usually first noticed in teenagers or young adults. There is strong evidence that narcolepsy may run in families; around 10 percent of people diagnosed with narcolepsy with cataplexy have a close relative with this neurological disorder. While narcolepsy symptoms are often confused with depression, there is a link between the two disorders. Research studies have mixed results on co-occurrence of depression in narcolepsy patients - the numbers quoted by different studies are anywhere between 6% and 50%.

Narcolepsy has its typical onset in adolescence and young adulthood. There is an average 15-year delay between onset and correct diagnosis which may contribute substantially to the disabling features of the disorder. Cognitive, educational, occupational, and psychosocial problems associated with the excessive daytime sleepiness of narcolepsy have been documented. For these to occur in the crucial teen years when education, development of self-image, and development of occupational choice are taking place is especially devastating. While cognitive impairment does occur, it may only be a reflection of the excessive daytime somnolence.

The prevalence of narcolepsy is about 1 per 2,000 persons. It is a reason for patient visits to sleep disorder centers, and with its onset in adolescence, it is also a major cause of learning difficulty and absenteeism from school. Normal teenagers often already experience excessive daytime sleepiness because of a maturational increase in physiological sleep tendency accentuated by multiple educational and social pressures; this may be disabling with the addition of narcolepsy symptoms in susceptible teenagers. In clinical practice, the differentiation between narcolepsy and other conditions characterized by excessive somnolence may be difficult. Treatment options are currently limited. There is a paucity in the literature of controlled double-blind studies of possible effective drugs or other forms of therapy. Mechanisms of action of some few available therapeutic agents have been explored but detailed studies of mechanisms of action are needed before new classes of therapeutic agents can be developed. Narcolepsy is an underdiagnosed condition in the general population. This is partly because its severity varies, so it can be mistaken for other illnesses very easily. Some people with narcolepsy do not suffer from loss of muscle control.

At least one clinical trial on readaptation of the vestibulo-ocular reflex undertaken by Dr Mingjia Dai from Mount Sinai Hospital in New York City has produced results for a significant percentage of patients who have participated in the program.

Dai has developed an intervention that provided improvement in symptoms for 70% of the patients in the clinical trial phase. The protocol involves a physical manipulation of the patient intended to readapt the vestibulo-ocular reflex. While the program is no longer in the research phase, Dai continues to accept patients. According to Dai, "success" is measured as a 50% reduction of symptoms.

Recent research reveals a very small percentage of MdDS cases may be related to optokinetic nystagmus (OKN).