The most common sleep disorders include:

- Bruxism, involuntarily grinding or clenching of the teeth while sleeping.

- Catathrenia, nocturnal groaning during prolonged exhalation.

- Delayed sleep phase disorder (DSPD), inability to awaken and fall asleep at socially acceptable times but no problem with sleep maintenance, a disorder of circadian rhythms. Other such disorders are advanced sleep phase disorder (ASPD), non-24-hour sleep–wake disorder (non-24) in the sighted or in the blind, and irregular sleep wake rhythm, all much less common than DSPD, as well as the situational shift work sleep disorder.

- Hypopnea syndrome, abnormally shallow breathing or slow respiratory rate while sleeping.

- Idiopathic hypersomnia, a primary, neurologic cause of long-sleeping, sharing many similarities with narcolepsy.

- Insomnia disorder (primary insomnia), chronic difficulty in falling asleep and/or maintaining sleep when no other cause is found for these symptoms. Insomnia can also be comorbid with or secondary to other disorders.

- Kleine–Levin syndrome, a rare disorder characterized by persistent episodic hypersomnia and cognitive or mood changes.

- Narcolepsy, including excessive daytime sleepiness (EDS), often culminating in falling asleep spontaneously but unwillingly at inappropriate times. About 70% of those who have narcolepsy also have cataplexy, a sudden weakness in the motor muscles that can result in collapse to the floor while retaining full conscious awareness.

- Night terror, "Pavor nocturnus", sleep terror disorder, an abrupt awakening from sleep with behavior consistent with terror.

- Nocturia, a frequent need to get up and urinate at night. It differs from enuresis, or bed-wetting, in which the person does not arouse from sleep, but the bladder nevertheless empties.

- Parasomnias, disruptive sleep-related events involving inappropriate actions during sleep, for example sleep walking, night-terrors and catathrenia.

- Periodic limb movement disorder (PLMD), sudden involuntary movement of arms and/or legs during sleep, for example kicking the legs. Also known as nocturnal myoclonus. See also Hypnic jerk, which is not a disorder.

- Rapid eye movement sleep behavior disorder (RBD), acting out violent or dramatic dreams while in REM sleep, sometimes injuring bed partner or self (REM sleep disorder or RSD).

- Restless legs syndrome (RLS), an irresistible urge to move legs. RLS sufferers often also have PLMD.

- Shift work sleep disorder (SWSD), a situational circadian rhythm sleep disorder. (Jet lag was previously included as a situational circadian rhythm sleep disorder, but it doesn't appear in DSM-5 (see Diagnostic and Statistical Manual of Mental Disorders)).

- Sleep apnea, obstructive sleep apnea, obstruction of the airway during sleep, causing lack of sufficient deep sleep, often accompanied by snoring. Other forms of sleep apnea are less common. When air is blocked from entering into the lungs, the individual unconsciously gasps for air and sleep is disturbed. Stops of breathing of at least ten seconds, 30 times within seven hours of sleep, classifies as apnea. Other forms of sleep apnea include central sleep apnea and sleep-related hypoventilation.

- Sleep paralysis, characterized by temporary paralysis of the body shortly before or after sleep. Sleep paralysis may be accompanied by visual, auditory or tactile hallucinations. Not a disorder unless severe. Often seen as part of narcolepsy.

- Sleepwalking or "somnambulism", engaging in activities normally associated with wakefulness (such as eating or dressing), which may include walking, without the conscious knowledge of the subject.

- Somniphobia, one cause of sleep deprivation, a dread/ fear of falling asleep or going to bed. Signs of the illness include anxiety and panic attacks before and during attempts to sleep.

Confusional arousal has a prevalence of 4% in adults, and is common in children. Infants and toddlers usually experience confusional arousals beginning with large amounts of movement and moaning, which can later progress to occasional thrashings or inconsolable crying. Confusional arousal is a condition when an individual awakens from sleep and remains in a confused state. It is characterized by the individual's partial awakening and sitting up to look around. They usually remain in bed and then return to sleep. These episodes last anywhere from seconds to minutes and may not be reactive to stimuli.

Confusional arousals are not considered dangerous. Another sleeping disorder may be present triggering these incomplete arousals.

There are two main characteristics of narcolepsy: excessive daytime sleepiness and abnormal REM sleep. The first, excessive daytime sleepiness (EDS), occurs even after adequate night time sleep. A person with narcolepsy is likely to become drowsy or fall asleep, often at inappropriate times and places, or just be very tired throughout the day. Narcoleptics are not able to experience the amount of restorative deep sleep that healthy people experience – they are not "over-sleeping". In fact, narcoleptics live their entire lives in a constant state of extreme sleep deprivation.

Daytime naps may occur with little warning and may be physically irresistible. These naps can occur several times a day. They are typically refreshing, but only for a few hours or less. Vivid dreams may be experienced on a regular basis, even during very brief naps. Drowsiness may persist for prolonged periods or remain constant. In addition, night-time sleep may be fragmented, with frequent awakenings. A second prominent symptom of narcolepsy is abnormal REM sleep. Narcoleptics are unique in that they enter into the REM phase of sleep in the beginnings of sleep, even when sleeping during the day.

The classic symptoms of the disorder, often referred to as the "tetrad of narcolepsy," are cataplexy, sleep paralysis, hypnagogic hallucinations, and excessive daytime sleepiness. Other symptoms may include automatic behaviors and night-time wakefulness. These symptoms may not occur in all patients.

- Cataplexy is an episodic loss of muscle function, ranging from slight weakness such as limpness at the neck or knees, sagging facial muscles, weakness at the knees often referred to as "knee buckling", or inability to speak clearly, to a complete body collapse. Episodes may be triggered by sudden emotional reactions such as laughter, anger, surprise, or fear, and may last from a few seconds to several minutes. The person remains conscious throughout the episode. In some cases, cataplexy may resemble epileptic seizures. Usually speech is slurred and vision is impaired (double vision, inability to focus), but hearing and awareness remain normal. Cataplexy also has a severe emotional impact on narcoleptics, as it can cause extreme anxiety, fear, and avoidance of people or situations that might elicit an attack. Cataplexy is generally considered to be unique to narcolepsy and is analogous to sleep paralysis in that the usually protective paralysis mechanism occurring during sleep is inappropriately activated. The opposite of this situation (failure to activate this protective paralysis) occurs in rapid eye movement behavior disorder.

- Periods of wakefulness at night

- Sleep paralysis is the temporary inability to talk or move when waking (or less often, when falling asleep). It may last a few seconds to minutes. This is often frightening but is not dangerous.

- Hypnagogic hallucinations are vivid, often frightening, dreamlike experiences that occur while dozing or falling asleep. Hypnopompic hallucinations refer to the same sensations while awakening from sleep. These hallucinations may manifest in the form of visual or auditory sensations.

- Automatic behaviors occur when a person continues to function (talking, putting things away, etc.) during sleep episodes but awakens with no memory of performing such activities. It is estimated that up to 40 percent of people with narcolepsy experience automatic behavior during sleep episodes.

In most cases, the first symptom of narcolepsy to appear is excessive and overwhelming daytime sleepiness. The other symptoms may begin alone or in combination months or years after the onset of the daytime naps. There are wide variations in the development, severity, and order of appearance of cataplexy, sleep paralysis, and hypnagogic hallucinations in individuals. Only about 20 to 25 percent of people with narcolepsy experience all four symptoms. The excessive daytime sleepiness generally persists throughout life, but sleep paralysis and hypnagogic hallucinations may not. A rare subset of narcoleptics also experience a heightened sense of taste and smell known as the supertaster phenomenon.

Many people with narcolepsy also suffer from insomnia for extended periods of time. The excessive daytime sleepiness and cataplexy often become severe enough to cause serious problems in a person's social, personal, and professional life. Normally, when an individual is awake, brain waves show a regular rhythm. When a person first falls asleep, the brain waves become slower and less regular, which is called non-rapid eye movement (NREM) sleep. After about an hour and a half of NREM sleep, the brain waves begin to show a more active pattern again, called REM sleep (rapid eye movement sleep), when most remembered dreaming occurs. Associated with the EEG-observed waves during REM sleep, muscle atonia is present called REM atonia.

In narcolepsy, the order and length of NREM and REM sleep periods are disturbed, with REM sleep occurring at sleep onset instead of after a period of NREM sleep. Also, some aspects of REM sleep that normally occur only during sleep, like lack of muscular control, sleep paralysis, and vivid dreams, occur at other times in people with narcolepsy. For example, the lack of muscular control can occur during wakefulness in a cataplexy episode; it is said that there is intrusion of REM atonia during wakefulness. Sleep paralysis and vivid dreams can occur while falling asleep or waking up. Simply put, the brain does not pass through the normal stages of dozing and deep sleep but goes directly into (and out of) rapid eye movement (REM) sleep.

As a consequence night time sleep does not include as much deep sleep, so the brain tries to "catch up" during the day, hence EDS. People with narcolepsy may visibly fall asleep at unpredicted moments (such motions as head bobbing are common). People with narcolepsy fall quickly into what appears to be very deep sleep, and they wake up suddenly and can be disoriented when they do (dizziness is a common occurrence). They have very vivid dreams, which they often remember in great detail. People with narcolepsy may dream even when they only fall asleep for a few seconds. Along with vivid dreaming, people with narcolepsy are known to have audio or visual hallucinations prior to falling asleep.

Narcoleptics can gain excess weight; children can gain 20 to 40 lb (9 to 18 kg) when they first develop narcolepsy; in adults the body-mass index is about 15% above average.

NREM parasomnias are arousal disorders that occur during stage 3 (or 4 by the R&K standardization) of NREM sleep—also known as slow wave sleep (SWS). They are caused by a physiological activation in which the patient’s brain exits from SWS and is caught in between a sleeping and waking state. In particular, these disorders involve activation of the autonomic nervous system, motor system, or cognitive processes during sleep or sleep-wake transitions.

Some NREM parasomnias (sleep-walking, night-terrors, and confusional arousal) are common during childhood but decrease in frequency with increasing age. They can be triggered in certain individuals, by alcohol, sleep deprivation, physical activity, emotional stress, depression, medications, or a fevered illness. These disorders of arousal can range from confusional arousals, somnambulism, to night terrors. Other specific disorders include sleepeating, sleep sex, teeth grinding, rhythmic movement disorder, restless legs syndrome, and somniloquy.

A sleep diary should be kept to aid in diagnosis and for chronicling the sleep schedule during treatment. Other ways to monitor the sleep schedule are actigraphy or use of a Continuous Positive Airway Pressure (CPAP) machine that can log sleeping times

The following are possible warning signs:

- sleeping off and on in a series of naps during the day and at night, with no regular pattern but with normal total sleep time,

- difficulty getting restorative sleep, and

- excessive daytime sleepiness.

Because of the changes in sleep/wake time, and because this is a rare disorder, initially it can seem like another circadian rhythm sleep disorder such as non-24-hour sleep–wake disorder or like insomnia.

There are over 30 recognized kinds of dyssomnias. Major groups of dyssomnias include:

- Intrinsic sleep disorders – 12 disorders recognized, including

- idiopathic hypersomnia,

- narcolepsy,

- periodic limb movement disorder,

- restless legs syndrome,

- sleep apnea,

- sleep state misperception.

- Extrinsic sleep disorders – 13 disorders recognized, including

- alcohol-dependent sleep disorder,

- food allergy insomnia,

- inadequate sleep routine.

- Circadian rhythm sleep disorders, both intrinsic and extrinsic – 6 disorders recognized, including

- advanced sleep phase syndrome,

- delayed sleep phase syndrome,

- jetlag,

- shift work sleep disorder.

RBD is characterized by the dreamer acting out his or her dreams. These dreams often involve kicking, screaming, punching, grabbing, and even jumping out of bed. When awakened, people can usually recall the dream they were having, which will match the actions they were performing, but they will not be aware that they were moving. In a normal sleep cycle, REM sleep may be experienced at intervals of between 90 minutes and two hours every night, which means RBD episodes may occur up to four times a night. In a rare case, they may only happen once a week or once a month. Episodes occur more towards the morning hours because that is when REM sleep is more frequent. The actions in an episode can result in injuries to oneself or one's bed partner. People can also respond to other people while sleeping and not even know it. This causes them to be aware of things while they are sleeping, which can result in sleep deprivation.

The current formally correct name of the disorder is Circadian Rhythm Sleep Disorder: Irregular Sleep Wake Rhythm Type. This disorder has been referred to by many other terms, including: Irregular Sleep Wake Pattern, irregular sleep wake syndrome, Irregular Sleep Wake Rhythm (ISWRD), Irregular Sleep Wake Cycle, Irregular Sleep Wake Schedule and Irregular Sleep Wake Disorder (ISWD). Sometimes the words sleep and wake are hyphenated (sleep–wake). Sometimes the words are capitalized and sometimes they are not.

The universal feature of night terrors is inconsolability, very similar to that of a panic attack. During night terror bouts, people are usually described as "bolting upright" with their eyes wide open and a look of fear and panic on their faces. They will often scream. Furthermore, they will usually sweat, exhibit rapid breathing, and have a rapid heart rate (autonomic signs). In some cases, individuals are likely to have even more elaborate motor activity, such as a thrashing of limbs—which may include punching, swinging, or fleeing motions. There is a sense that the individuals are trying to protect themselves and/or escape from a possible threat of bodily injury. Although people may seem to be awake during a night terror, they will appear confused, be inconsolable and/or unresponsive to attempts to communicate with them, and may not recognize others familiar to them. Occasionally, when a person with a night terror is awakened, they will lash out at the one awakening them, which can be dangerous to that individual. Most people who experience this do not remember the incident the next day. Sleepwalking is also common during night terror bouts, as sleepwalking and night terrors are different manifestations of the same parasomnia.

During lab tests, subjects are known to have very high voltages of electroencephalography (EEG) delta activity, an increase in muscle tone, and a doubled increase in heart rate, if not more. Brain activities during a typical episode show theta and alpha activity when using an EEG. It is also common to see abrupt arousal from NREM sleep that does not progress into a full episode of a night terror. These episodes can include tachycardia. Night terrors are also associated with intense autonomic discharge of tachypnea, flushing, diaphoresis, and mydriasis – that is, unconscious or involuntary rapid breathing, reddening of the skin, profuse sweating, and dilation of the pupils.

In children with night terrors, there is no increased occurrence of psychiatric diagnoses. However, in adults who suffer from night terrors there is a close association with psychopathology or mental disorders. There may be an increased occurrence of night terrors—particularly among those suffering or having suffered from post-traumatic stress disorder (PTSD) and generalized anxiety disorder (GAD). It is also likely that some personality disorders may occur in individuals with night terrors, such as dependent, schizoid, and borderline personality disorders. There have been some symptoms of depression and anxiety that have increased in individuals that have suffered from frequent night terrors. Low blood sugar is associated with both pediatric and adult night terrors. A study of adults with thalamic lesions of the brain and brainstem have been occasionally associated with night terrors. Night terrors are closely linked to sleepwalking and frontal lobe epilepsy.

Hypersomnia can be primary (of central/brain origin), or it can be secondary to any of numerous medical conditions. More than one type of hypersomnia can coexist in a single patient. Even in the presence of a known cause of hypersomnia, the contribution of this cause to the complaint of EDS needs to be assessed. When specific treatments of the known condition do not fully suppress EDS, additional causes of hypersomnia should be sought. For example, if a patient with sleep apnea is treated with CPAP (continuous positive airway pressure) which resolves their apneas but not their EDS, it is necessary to seek other causes for the EDS. Obstructive sleep apnea “occurs frequently in narcolepsy and may delay the diagnosis of narcolepsy by several years and interfere with its proper management.”

The true primary hypersomnias include these: narcolepsy (with and without cataplexy); idiopathic hypersomnia; and recurrent hypersomnias (like Klein-Levin syndrome).

Those who suffer from idiopathic hypersomnia have recurring episodes of excessive daytime sleepiness (EDS). These occur in spite of "adequate, or more typically, extraordinary sleep amounts (e.g., greater than 10 hours per night)." Sleep is usually deep, with significant difficulty arousing from sleep, even with use of several alarm clocks. In fact, patients with IH often must develop elaborate rituals to wake, as alarm clocks and even physical attempts by friends/family to wake them may fail. Despite getting more hours of sleep than typically required by the human body, patients awake unrefreshed and may also suffer sleep inertia, known more descriptively in its severe form as sleep drunkenness (significant disorientation upon awakening). Daytime naps are generally very long (up to several hours) and are also unrefreshing, as opposed to the short refreshing naps associated with narcolepsy. Sleep paralysis and hypnagogic hallucinations may also occur, as well as motor hyper-reactivity.

Several studies have shown increased frequencies of other symptoms in patients with idiopathic hypersomnia, although it is not clear whether these symptoms are caused by the idiopathic hypersomnia. These symptoms include palpitations, digestive problems, difficulty with body temperature regulation, and cognitive problems, especially deficits in memory, attention, and concentration. Anxiety and depression are often increased in idiopathic hypersomnia, most likely as a response to chronic illness. A large case series in 2010 found that peripheral vascular symptoms, such as cold hands and feet (Raynaud’s-type phenomena) were significantly more common in people with idiopathic hypersomnia than in controls. In addition to difficulty with temperature regulation and Raynaud’s type symptoms, other symptoms associated with autonomic dysfunction were noted to occur in idiopathic hypersomnia. These included: fainting episodes (syncope); dizziness upon arising (orthostatic hypotension); and headaches (possibly migrainous in quality). Food cravings and impotence have also been reported.

Symptom intensity often varies between weeks, months, or years, and symptoms can worsen just prior to menses in women. Many patients are chronically tardy to work, school or social engagements and, over time, may lose the ability to function in family, social, occupational or other settings altogether. (See Prognosis section below).

A systematic review found that traumatic childhood experiences (such as family conflict or sexual trauma) significantly increases the risk for a number of sleep disorders in adulthood, including sleep apnea, narcolepsy, and insomnia. It is currently unclear whether or not moderate alcohol consumption increases the risk of obstructive sleep apnea.

In addition, an evidence-based synopses suggests that the sleep disorder, idiopathic REM sleep behavior disorder (iRBD), may have a hereditary component to it. A total of 632 participants, half with iRBD and half without, completed self-report questionnaires. The results of the study suggest that people with iRBD are more likely to report having a first-degree relative with the same sleep disorder than people of the same age and sex that do not have the disorder. More research needs to be conducted to gain further information about the hereditary nature of sleep disorders.

A population susceptible to the development of sleep disorders is people who have experienced a traumatic brain injury (TBI). Because many researchers have focused on this issue, a systematic review was conducted to synthesize their findings. According to their results, TBI individuals are most disproportionately at risk for developing narcolepsy, obstructive sleep apnea, excessive daytime sleepiness, and insomnia. The study's complete findings can be found in the table below:

This sleep disorder frequently applies when patients report not feeling tired despite their subjective perception of not having slept. Generally, they may describe experiencing several years of no sleep, short sleep, or non-restorative sleep. Otherwise, patients appear healthy, both psychiatrically and medically. (That this condition is often asymptomatic could explain why it is relatively unreported.)

However, upon clinical observation, it is found that patients may severely overestimate the time they took to fall asleep—often reporting having slept half the amount of time indicated by polysomnogram or electroencephalography (EEG), which may record normal sleep. Observing such discrepancy between subjective and objective reports, clinicians may conclude that the perception of poor sleep is primarily illusionary.

Alternatively, some people may report excessive daytime sleepiness or chronic disabling sleepiness, while no sleep disorder has been found to exist. Methods of diagnosing sleepiness objectively, such as the Multiple Sleep Latency Test do not confirm the symptom"true" sleepiness is not observed despite the complaint. (It may be speculated that such reports of daytime sleepiness may be a result of the nocebo response —the reverse of the placebo effect—due to patient expectations of adverse effects from their subjective perception of poor sleep.)

Finally, on the opposite end of the spectrum, other patients may report feeling that they have slept much longer than is observed. It has been proposed that this experience be subclassified under sleep state misperception as "positive sleep state misperception", "reverse sleep state misperception", and "negative sleep state misperception".

Four of them are intrinsic (from Latin "intrinsecus", on the inside, inwardly), "built-in":

- Advanced sleep phase disorder (ASPD), a.k.a. advanced sleep phase syndrome (ASPS), characterized by difficulty staying awake in the evening and difficulty staying asleep in the morning

- Delayed sleep phase disorder (DSPD), a.k.a. delayed sleep phase syndrome (DSPS), characterized by a much later than normal timing of sleep onset and offset and a period of peak alertness in the middle of the night

- Irregular sleep–wake rhythm, which presents as sleeping at very irregular times, and usually more than twice per day (waking frequently during the night and taking naps during the day) but with total time asleep typical for the person's age

- Non-24-hour sleep–wake disorder (non-24, a.k.a. hypernychthemeral syndrome), in which the affected individual's sleep occurs later and later each day, with the period of peak alertness also continuously moving around the clock from day to day.

The central symptom of sleep paralysis is being aware but being unable to move during awakening.

Imagined sounds such as humming, hissing, static, zapping and buzzing noises are reported during sleep paralysis. Other sounds such as voices, whispers and roars are also experienced. These symptoms are usually accompanied by intense emotions: such as fear, and panic. People also have sensations of being dragged out of bed or of flying, numbness, and feelings of electric tingles or vibrations running through their body.

Sleep paralysis may include hallucinations, such as a supernatural creature suffocating or terrifying the individual, accompanied by a feeling of pressure on one's chest and difficulty breathing. Another example of a hallucination involves a menacing shadowy figure entering one's room or lurking outside one's window, while the subject is paralyzed.

The body image distortion (affecting parietal regions and the temporoparietal junction) may result in the sleeper having bodily hallucinations, such as illusory limbs and out-of-body experiences. The content and interpretation of these hallucinations are driven by fear, somatic sensations, REM-induced sexual arousal, and REM mentation which are embedded in the sleeper's cultural narrative.

REM sleep physiology and somatic symptoms coupled with the awareness that one is paralyzed, can generate a variety of psychological symptoms during sleep paralysis, including fear and worry that are aggravated by catastrophic cognitions about the attack. This can activate a fight-flight reaction and panic-like arousal. Consequently, when the person attempts to escape the paralysis, somatic symptoms and arousal are exacerbated, as execution of motor programs in the absence of dampening proprioceptive feedback can lead to heightened sensations of bodily tightness and pressure, and even pain and spasms in limbs.

Excessive daytime sleepiness (EDS) is characterized by persistent sleepiness and often a general lack of energy, even during the day after apparently adequate or even prolonged nighttime sleep. EDS can be considered as a broad condition encompassing several sleep disorders where increased sleep is a symptom, or as a symptom of another underlying disorder like narcolepsy, sleep apnea or a circadian rhythm sleep disorder.

Some persons with EDS, including those with hypersomnias like narcolepsy and idiopathic hypersomnia, are compelled to nap repeatedly during the day; fighting off increasingly strong urges to sleep during inappropriate times such as while driving, while at work, during a meal, or in conversations. As the compulsion to sleep intensifies, the ability to complete tasks sharply diminishes, often mimicking the appearance of intoxication. During occasional unique and/or stimulating circumstances, a person with EDS can sometimes remain animated, awake and alert, for brief or extended periods of time. EDS can affect the ability to function in family, social, occupational, or other settings. A proper diagnosis of the underlying cause and ultimately treatment of symptoms and/or the underlying cause can help mitigate such complications.

Nocturnal awakenings are more common in older patients and have been associated with depressive disorders, chronic pain, obstructive sleep apnea, obesity, alcohol consumption, hypertension, gastroesophageal reflux disease, heart disease, menopause, prostate problems, and bipolar disorders.

Nocturnal awakenings can be mistaken as shift work disorder.

Rapid eye movement behavior disorder (RBD) occurs when there is a loss of normal voluntary muscle atonia during REM sleep resulting in motor behavior in response to dream content. It can be caused by adverse reactions to certain drugs or during drug withdrawal; however, it is most often associated with the elderly and in those with neurodegenerative disorders such as Parkinson disease and other neurodegenerative diseases, for example multiple system atrophy and Lewy body dementia.

RBD is categorized as either idiopathic or symptomatic.

Night terrors typically occur in children between the ages of three and twelve years, with a peak onset in children aged three and a half years old.

An estimated 1–6% of children experience night terrors. Boys and girls of all ethnic backgrounds are affected equally. In children younger than three and a half years old, peak frequency of night terrors is at least one episode per week. Among older children, peak frequency of night terrors is one or two episodes per month. The children will most likely have no recollection of the episode the next day. Pediatric evaluation may be sought to exclude the possibility that the night terrors are caused by seizure disorders or breathing problems. Most children will outgrow sleep terrors.

Episodes of sleep paralysis can occur in the context of several medical conditions (e.g., narcolepsy, hypokalemia). When episodes occur independent of these conditions or substance use, it is termed "isolated sleep paralysis" (ISP). When ISP episodes are more frequent and cause clinically-significant distress and/or interference, it is classified as "recurrent isolated sleep paralysis"(RISP). Episodes of sleep paralysis, regardless of classification, are generally short (1–6 minutes), but longer episodes have been documented. With RISP the individual can also suffer back-to-back episodes of sleep paralysis in the same night, which is unlikely in individuals who suffer from ISP.

It can be difficult to differentiate between cataplexy brought on by narcolepsy and true sleep paralysis, because the two phenomena are physically indistinguishable. The best way to differentiate between the two is to note when the attacks occur most often. Narcolepsy attacks are more common when the individual is falling asleep; ISP and RISP attacks are more common upon awakening.

The patient has a complaint of insomnia while sleep quality and duration are normal. Polysomnographic monitoring demonstrates normal sleep latency, a normal number of arousals and awakenings, and normal sleep duration with or without a multiple sleep latency test that demonstrates a mean sleep latency of greater than 10 minutes. No medical or mental disorder produces the complaint. Other sleep disorders producing insomnia are not present to a degree that would explain the patient’s complaint.

One of these disorders is extrinsic (from Latin "extrinsecus", from without, on the outside) or circumstantial:

- Shift work sleep disorder, which affects people who work nights or rotating shifts.

Formerly, jet lag, too, was classified as an extrinsic type circadian rhythm disorder.

In addition to differentiating between the primary and secondary hypersomnias, the 2001 International Classification of Sleep Disorders (ICSD) further classified the primary hypersomnia syndromes. These included idiopathic hypersomnia, narcolepsy, and the recurrent hypersomnias (like Klein-Levin syndrome).

The 2001 ICSD defines idiopathic hypersomnia as "a disorder of presumed central nervous system cause that is associated with a normal or prolonged major sleep episode and excessive sleepiness consisting of prolonged (1- to 2-hour) sleep episodes of N-REM"(non-rapid eye movement sleep). The ICSD initially described two clinical forms of idiopathic hypersomnia: "1) a polysymptomatic form with nocturnal sleep and naps of abnormally long duration with ‘sleep drunkenness’ on awakening, and 2) a monosymptomatic form manifested by isolated EDS." These forms were later described as idiopathic hypersomnia with long sleep time and idiopathic hypersomnia without long sleep time, respectively.

This classification has steadily evolved, as further research has shown overlap between narcolepsy and idiopathic hypersomnia. The 3rd edition of the ICSD labels narcolepsy caused by hypocretin deficiency as "type 1 narcolepsy," which is almost always associated with cataplexy. The other hypersomnias remain subdivided based on the presence of sleep-onset rapid eye movement periods (SOREMPs). They are labeled: "type 2 narcolepsy," with 2 or more SOREMPs on mean sleep latency testing (MSLT); and "idiopathic hypersomnia," with less than 2 SOREMPS.

However, "there is no evidence that the pathophysiology or therapeutic response is substantially different for hypersomnia with or without SOREMPs on the MSLT." Given the newly understood overlap of idiopathic hypersomnia and narcolepsy, the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders also updated its classification of the primary hypersomnias. It reclassified both idiopathic hypersomnia with and without long sleep time as major somnolence disorder (MSD). Additionally, MSD encompasses all syndromes of hypersomnolence not explained by low hypocretin, including not only idiopathic hypersomnia, but also narcolepsy without cataplexy, and long sleepers (patients requiring >10 hours sleep/day).

Further complicating these updated classification schemes, overlap between narcolepsy with cataplexy and idiopathic hypersomnia has also been reported. A subgroup of narcoleptics with long sleep time, comprising 18% of narcoleptics in one study, had symptoms of both narcolepsy with cataplexy and idiopathic hypersomnia (long sleep time and unrefreshing naps). It is felt that this subgroup might have dysfunction in multiple arousal systems. (See Causes section below).

People with sleep apnea have problems with excessive daytime sleepiness (EDS), impaired alertness, and vision problems. OSA may increase risk for driving accidents and work-related accidents. If OSA is not treated, people are at increased risk of other health problems, such as diabetes. Death could occur from untreated OSA due to lack of oxygen to the body. Moreover, people are examined using "standard test batteries" to further identify parts of the brain that may be adversely affected by sleep apnea, including those that govern:

- "executive functioning", the way the person plans and initiates tasks

- paying attention, working effectively and processing information when in a waking state

- using memory and learning.

Due to the disruption in daytime cognitive state, behavioral effects may be present. These can include moodiness, belligerence, as well as a decrease in attentiveness and energy. These effects may become intractable, leading to depression.

There is evidence that the risk of diabetes among those with moderate or severe sleep apnea is higher. There is increasing evidence that sleep apnea may lead to liver function impairment, particularly fatty liver diseases (see steatosis). Finally, because there are many factors that could lead to some of the effects previously listed, some people are not aware that they have sleep apnea and are either misdiagnosed or ignore the symptoms altogether.