In patients who are at high likelihood of having OSA, a randomized controlled trial found that home oximetry (a non-invasive method of monitoring blood oxygenation) may be adequate and easier to obtain than formal polysomnography. High probability patients were identified by an Epworth Sleepiness Scale (ESS) score of 10 or greater and a Sleep Apnea Clinical Score (SACS) of 15 or greater. Home oximetry, however, does not measure apneic events or respiratory event-related arousals and thus does not produce an AHI value.

A physician specializing in sleep medicine may ask patients about their medical history; for example: neurological problems, prescription or non-prescription medications taken, alcohol use, family history, and any other sleep problems. A thorough medical and neurological exam is indicated. The patient will be asked to complete a sleep diary, recording natural sleep and wake up times, over several weeks. Sleep rating with the Epworth Sleepiness Scale may be used.

A neurological condition or another medical problem may be suspected, in which case, blood tests, a CT scan or an MRI may be used. An overnight sleep study is usually not needed to detect this disorder, but may be indicated if other sleep disorders, such as sleep apnea and periodic limb movement disorder, seem likely. The overnight sleep study is called polysomnography. It charts brain waves, heart beat, muscle activity, and breathing during sleep. It also records arm and leg movement. It will show if there are other sleep disorders that are causing or increasing the problems with ISWD.

Oximetry, which may be performed over one or several nights in a person's home, is a simpler, but less reliable alternative to a polysomnography. The test is only recommended when requested by a physician and should not be used to test those without symptoms. Home oximetry may be effective in guiding prescription for automatically self-adjusting continuous positive airway pressure.

Because a number of parasomnias may be confused with RBD, it is necessary to conduct formal sleep studies such as polysomnography (PSG) performed at sleep centers that are experienced in evaluating parasomnias in order to establish a diagnosis. In RBD, a single night of extensive monitoring of sleep, brain, and muscle activity will almost always reveal the lack of muscle paralysis during REM sleep, and it will also eliminate other causes of parasomnias.

Recently, due to the limited access to PSG, attempts have been made to identify RBD from clinical interview as well as questionnaires. Postuma et al. have validated a single-question screening tool for RBD (RBD1Q) that could be easily applied in general practice to the patient and their bed partner. A positive answer to the RBDQ1, ‘Have you ever been told or suspected yourself, that you seem to act out your dreams while asleep (for example, punching, flailing your arms in the air, making running movement etc.)?’ should encourage the medical practitioner to consider the diagnosis of RBD as it offers good sensitivity (94%) and specificity (87%). Other questionnaires, such as the Rapid Eye Movement (REM) sleep Behavior Disorder Screening Questionnaire (RBDSQ) or the REM Sleep Behavior Questionnaires – Hong-Kong are available for more detailed characterisation.

Idiopathic hypersomnia has historically been "difficult to diagnose at an early stage," especially because many other disorders can cause symptoms of excessive daytime sleepiness (EDS). Therefore, "at the time of presentation, most patients have had the disorder for many years."

Further complicating the diagnostic process, idiopathic hypersomnia lacks a clearly defining clinical feature. Whereas narcolepsy is associated with cataplexy and sleep-onset REM episodes, and Kleine-Levin syndrome is associated with megaphagia (compulsive food cravings) and hypersexuality, idiopathic hypersomnia has no such dramatic associated features, except perhaps sleep drunkenness. "Consequently there has been an unfortunate tendency to label all difficult-to-classify cases of excessive daytime sleepiness as idiopathic hypersomnia." For example, upper airway resistance syndrome and delayed sleep phase disorder were formerly confused with idiopathic hypersomnia, but now that they have been more clearly defined, doctors can more carefully exclude these causes of EDS in order to more correctly diagnose idiopathic hypersomnia. However, "even in the presence of other specific causes of hypersomnia, one should carefully assess the contribution of these etiological factors to the complaint of EDS and when specific treatments of these conditions fail to suppress EDS, the [additional] diagnosis of idiopathic hypersomnia should be considered."

The severity of EDS can be quantified by subjective scales, such as the Epworth sleepiness scale and the Stanford sleepiness scale (SSS), and also by objective tests, like the multiple sleep latency test (MSLT)."

In 2001, the ICSD (International Classification of Sleep Disorders) updated their criteria for the diagnosis of idiopathic hypersomnia. Essentially, EDS must be present for at least 6 months, sleep studies (polysomnography and multiple sleep latency test) must show certain characteristics, and all other known causes for long sleep time and EDS must be considered (see hypersomnia). For the patient, this diagnostic process is often tedious, expensive and time-consuming, as other than the sleep studies, it is still basically a diagnosis of exclusion.

In patients with idiopathic hypersomnia, polysomnography typically shows short sleep latency, increased mean slow wave sleep, and a high mean sleep efficiency. "Latency to REM sleep and percentages of light sleep and REM sleep were normal, compared with normal ranges." Despite this, one study has found increased sleep fragmentation in patients with idiopathic hypersomnia without long sleep time, suggesting multiple possible presentations.

It is important to note that although sleep latencies are typically short in idiopathic hypersomnia, the clinical severity may not correlate closely with the MSLT results. In fact, "latencies above 5 minutes are not uncommon in patients with clinically severe hypersomnia." When sleep latency is below 10 minutes, the presence of sleep-onset REM periods (SOREMPs) in two or more of the MSLT naps suggests a diagnosis of narcolepsy, whereas sleep periods lacking rapid eye movement (NREM sleep) in the various naps suggests a diagnosis of idiopathic hypersomnia. However, the importance of this differentiation between REM and NREM has been called into question. (see Classification)

Although the MSLT is currently the best available test to diagnose EDS in general, the MSLT protocol lacks the ability to document the extended, unrefreshing daytime naps that often occur in idiopathic hypersomnia. Complicating the matter, several groups of researchers have found normal MSLT results in patients who otherwise seem to have idiopathic hypersomnia. Therefore, when idiopathic hypersomnia is suspected, researchers suggest appending a 24-hour continuous polysomnography to the standard overnight/MSLT study in order to record total sleep time. Alternatively, an assay of the patient's cerebrospinal fluid (CSF) can be performed in order to test for an adequate level of hypocretin (to exclude narcolepsy with cataplexy) and to determine whether the patient’s CSF abnormally boosts GABA receptor sensitivity (thought to underlie many cases of idiopathic hypersomnia and narcolepsy without cataplexy). Globally, there are very few labs capable of performing the CSF assays referenced above.

It is also important to note that whereas narcolepsy is strongly associated with the HLA-DQB1*0602 genotype, "HLA typing is of no help in the positive diagnosis of idiopathic hypersomnia." This is "despite some reports that suggest an increase frequency of HLA Cw2 and DRS in idiopathic hypersomnia subjects."

Diagnosis is relatively easy when all the symptoms of narcolepsy are present, but if the sleep attacks are isolated and cataplexy is mild or absent, diagnosis is more difficult. It is also possible for cataplexy to occur in isolation. Three tests that are commonly used in diagnosing narcolepsy are the polysomnogram, the multiple sleep latency test (MSLT), and administration of the Epworth Sleepiness Scale. These tests are usually performed by a sleep specialist. The polysomnogram involves continuous recording of sleep brain waves and a number of nerve and muscle functions during night time sleep. When tested, people with narcolepsy fall asleep rapidly, enter REM sleep early, and may often awaken during the night. The polysomnogram also helps to detect other possible sleep disorders that could cause daytime sleepiness.

The Epworth Sleepiness Scale is a brief questionnaire that is administered to determine the likelihood of the presence of a sleep disorder, including narcolepsy. For the multiple sleep latency test, a person is given a chance to sleep every 2 hours during normal wake times. The patient is taken in usually for an overnight sleep study. The following day the patient will have multiple tests where they will be told to nap after a full nights sleep (usually eight hours). Observations are made of the time taken to reach various stages of sleep (sleep onset latency). This test measures the degree of daytime sleepiness and also detects how soon REM sleep begins. Again, people with narcolepsy fall asleep rapidly and enter REM sleep early. Occasionally, a multiple sleep latency test can result in a false-negative for a narcoleptic.

The system which regulates sleep, arousal, and transitions between these states in humans is composed of three interconnected subsystems: the orexin projections from the lateral hypothalamus, the reticular activating system, and the ventrolateral preoptic nucleus. In narcoleptic individuals, these systems are all associated with impairments due to a greatly reduced number of hypothalamic orexin projection neurons and significantly fewer orexin neuropeptides in cerebrospinal fluid and neural tissue, compared to non-narcoleptic individuals. Those with narcolepsy generally experience the REM stage of sleep within five minutes of falling asleep, while people who do not have narcolepsy (unless they are significantly sleep deprived) do not experience REM until after a period of slow-wave sleep, which lasts for about the first hour or so of a sleep cycle.

Measuring orexin levels in a person's cerebrospinal fluid sampled in a spinal tap may help in diagnosing narcolepsy, with abnormally low levels serving as an indicator of the disorder. This test can be useful when MSLT results are inconclusive or difficult to interpret.

The 2001 International Classification of Sleep Disorders (ICSD) divides primary hypersomnia syndromes between narcolepsy, idiopathic hypersomnia, and the recurrent hypersomnias (like Klein-Levin syndrome); it further divides narcolepsy into that with cataplexy and that without cataplexy. This ICSD version defines narcolepsy as a disorder of unknown cause "that is characterized by excessive sleepiness that typically is associated with cataplexy and other REM-sleep phenomena, such as sleep paralysis and hypnagogic hallucinations". It also establishes baseline categorical standards for diagnosis of narcolepsy, through 2 sets of well defined criteria, as follows.

Minimal narcolepsy diagnostic criteria set #2:

- A "complaint of excessive sleepiness or sudden muscle weakness."

- Associated features that include: sleep paralysis; disrupted major sleep episode; hypnagogic hallucinations; automatic behaviors.

- Polysomnography with one or more of the following: "sleep latency less than 10 minutes;" "REM sleep latency less than 20 minutes;" an MSLT with a mean sleep latency less than 5 minutes; "two or more sleep-onset REM periods" (SOREMPs).

- "No medical or mental disorder accounts for the symptoms." (see hypersomnia differential diagnosis)

In the absence of clear cataplexy, it becomes much more difficult to make a firm diagnosis of narcolepsy. “Various terms, such as essential hypersomnia, primary hypersomnia, ambiguous narcolepsy, atypical narcolepsy, etc., have been used to classify these patients, who may be in the developing phase of narcolepsy.”

Since the 2001 ICSD, the classification of primary hypersomnias has been steadily evolving, as further research has shown more overlap between narcolepsy and idiopathic hypersomnia. The 3rd edition of the ICSD is currently being finalized, and its new classification will label narcolepsy caused by orexin deficiency as “type 1 narcolepsy,” which is almost always associated with cataplexy. The other primary hypersomnias will remain subdivided based on the presence of SOREMPs. They will be labeled: “type 2 narcolepsy,” with 2 or more SOREMPs on 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 this currently understood overlap of idiopathic hypersomnia and narcolepsy, the 5th edition of the Diagnostic and Statistical Manual of Mental Disorders (DSM-V) is also updating its classification of the primary hypersomnias. It reclassifies narcolepsy without cataplexy as major somnolence disorder (MSD). Additionally, MSD will encompass all syndromes of hypersomnolence not explained by low orexin concentrations, including idiopathic hypersomnia (with and without long sleep time) and long sleepers (people 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 believed that this subgroup might have dysfunction in multiple arousal systems, including orexin and GABA (see idiopathic hypersomnia causes).

There are three types of sleep apnea. OSA accounts for 84%, CSA for 0.4%, and 15% of cases are mixed.

Polysomnography in diagnosing OSA characterizes the pauses in breathing. As in central apnea, pauses are followed by a relative decrease in blood oxygen and an increase in the blood carbon dioxide. Whereas in central sleep apnea the body's motions of breathing stop, in OSA the chest not only continues to make the movements of inhalation, but the movements typically become even more pronounced. Monitors for airflow at the nose and mouth demonstrate that efforts to breathe are not only present but that they are often exaggerated. The chest muscles and diaphragm contract and the entire body may thrash and struggle.

An "event" can be either an apnea, characterised by complete cessation of airflow for at least 10 seconds, or a hypopnea in which airflow decreases by 50 percent for 10 seconds or decreases by 30 percent if there is an associated decrease in the oxygen saturation or an arousal from sleep. To grade the severity of sleep apnea, the number of events per hour is reported as the apnea-hypopnea index (AHI). An AHI of less than 5 is considered normal. An AHI of 5-15 is mild; 15-30 is moderate and more than 30 events per hour characterizes severe sleep apnea.

Sleeping in a more upright position seems to lessen catathrenia (as well as sleep apnea). Performing regular aerobic exercise, where steady breathing is necessary (running, cycling etc.) may lessen catathrenia. Strength exercise, on the other hand, may worsen catathrenia because of the tendency to hold one's breath while exercising. Yoga and/or meditation focused on steady and regular breathing may lessen catathrenia.

Polysomnography is also used to aid in the diagnosis of other sleep disorders such as obstructive sleep apnea (OSA), narcolepsy, and restless leg syndrome (RLS). Normal test results show little to no episodes of sleep apnea and normal electrical activity in the individual's brain and muscles during sleep.

Several circumstances have been identified that are associated with an increased risk of sleep paralysis. These include insomnia, sleep deprivation, an erratic sleep schedule, stress, and physical fatigue. It is also believed that there may be a genetic component in the development of RISP, because there is a high concurrent incidence of sleep paralysis in monozygotic twins. Sleeping in the supine position has been found an especially prominent instigator of sleep paralysis.

Sleeping in the supine position is believed to make the sleeper more vulnerable to episodes of sleep paralysis because in this sleeping position it is possible for the soft palate to collapse and obstruct the airway. This is a possibility regardless of whether the individual has been diagnosed with sleep apnea or not. There may also be a greater rate of microarousals while sleeping in the supine position because there is a greater amount of pressure being exerted on the lungs by gravity.

While many factors can increase risk for ISP or RISP, they can be avoided with minor lifestyle changes. By maintaining a regular sleep schedule and observing good sleep hygiene, one can reduce chances of sleep paralysis. It helps subjects to reduce the intake of stimulants and stress in daily life by taking up a hobby or seeing a trained psychologist who can suggest coping mechanisms for stress. However, some cases of ISP and RISP involve a genetic factor—which means some people may find sleep paralysis unavoidable. Practicing meditation regularly might also be helpful in preventing fragmented sleep, and thus the occurrence of sleep paralysis. Research has shown that long-term meditation practitioners spend more time in slow wave sleep, and as such regular meditation practice could reduce nocturnal arousal and thus possibly sleep paralysis.

There have been some studies suggesting levothyroxine as a possible treatment for idiopathic hypersomnia, especially for patients with subclinical hypothyroidism. This treatment does carry potential risks (especially for patients without hypothyroidism or subclinical hypothroidism), which include cardiac arrhythmia.

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.

Polysomnography is a study conducted while the individual being observed is asleep. A polysomnograph (PSG) is a recording of an individual's body functions as they sleep. Complete sleep studies are most commonly facilitated at a designated sleep center. Specialized electrodes and monitors are connected to the individual and remain in place throughout study. Video cameras can be used in certain cases to record physical behaviors occurring while the individual is asleep. Typically the unwanted sexual behaviors do not present on film, therefore the majority of information is taken from a sleep study.

In most children, night terrors eventually subside and do not need to be treated. It may be helpful to reassure the child and their family that they will outgrow this disorder.

Psychotherapy or counseling can be helpful in many cases. There is some evidence to suggest that night terrors can result from lack of sleep or poor sleeping habits. In these cases, it can be helpful to improve the amount and quality of sleep which the child is getting. If this is not enough, benzodiazepines (such as diazepam) or tricyclic antidepressants may be used; however, medication is only recommended in extreme cases.

The "DSM-IV-TR" diagnostic criteria for sleep terror disorder requires:

- recurrent periods where the individual abruptly wakes from sleeping with a scream

- the individual experiences intense fear and symptoms of autonomic arousal, such as increased heart rate, heavy breathing, and increased perspiration

- the individual cannot be soothed or comforted during the episode

- the individual is unable to remember details of the dream or details of the episode

- the occurrence of the sleep terror episode causes "clinically significant" distress or impairment in the individual's functioning

- the disturbance is not due to the effects of a substance or general medical condition

Although "there has been no cure of chronic hypersomnia", there are several treatments that may improve patients' quality of life, depending on the specific cause or causes of hypersomnia that are diagnosed.

Diagnosis of Rhythmic Movement Disorder is done on an exclusionary basis in which other closely related movement disorders are systematically ruled out. Because of this, a thorough clinical evaluation is necessary. Often, impairments are not severe enough to warrant this process and so RMD is not often diagnosed unless there are extremely interfering or disabling symptoms. Many patients do not seek treatment for RMD directly and most seek professional help to alleviate sleep-affecting symptoms. To compound the issue, many sufferers are often misdiagnosed as having Restless Legs Syndrome or sleep apnea or some combination of the two. Rhythmic Movement Disorder differs from Restless Legs Syndrome in that RMD involves involuntary contractions of muscles with no urge or uncomfortable sensation to provoke such movement. Additionally, 80-90% of Restless Legs Syndrome sufferers show periodic limb movements as observed on a polysomnogram, which are not common in RMD patients. Rhythmic Movement Disorder can also have symptoms that overlap with epilepsy. However, use of a polysomnogram can help distinguish one disorder from the other as RMD involves movements in both REM and NREM sleep, which is unusual for seizures

. Additionally, patients can usually stop the movements upon request, unlike the movements observed in epilepsy. Other movement disorders like Parkinson’s Disease, Huntington’s Disease, ataxia, and dystonia differ from RMD in that they occur primarily during wakefulness and reduced sleep, whereas RMD episodes occur in or around sleep

"The severity of daytime sleepiness needs to be quantified by subjective scales (at least the Epworth Sleepiness Scale) and objective tests such as the multiple sleep latency test (MSLT)." The Stanford sleepiness scale (SSS) is another frequently-used subjective measurement of sleepiness. After it is determined that EDS is present, a complete medical examination and full evaluation of potential disorders in the differential diagnosis (which can be tedious, expensive and time-consuming) should be undertaken.

Treatments for sleep disorders generally can be grouped into four categories:

- Behavioral and psychotherapeutic treatment

- Rehabilitation and management

- Medication

- Other somatic treatment

None of these general approaches is sufficient for all patients with sleep disorders. Rather, the choice of a specific treatment depends on the patient's diagnosis, medical and psychiatric history, and preferences, as well as the expertise of the treating clinician. Often, behavioral/psychotherapeutic and pharmacological approaches are not incompatible and can effectively be combined to maximize therapeutic benefits. Management of sleep disturbances that are secondary to mental, medical, or substance abuse disorders should focus on the underlying conditions.

Medications and somatic treatments may provide the most rapid symptomatic relief from some sleep disturbances. Certain disorders like narcolepsy, are best treated with prescription drugs such as Modafinil. Others, such as chronic and primary insomnia, may be more amenable to behavioral interventions, with more durable results.

Chronic sleep disorders in childhood, which affect some 70% of children with developmental or psychological disorders, are under-reported and under-treated. Sleep-phase disruption is also common among adolescents, whose school schedules are often incompatible with their natural circadian rhythm. Effective treatment begins with careful diagnosis using sleep diaries and perhaps sleep studies. Modifications in sleep hygiene may resolve the problem, but medical treatment is often warranted.

Special equipment may be required for treatment of several disorders such as obstructive apnea, the circadian rhythm disorders and bruxism. In these cases, when severe, an acceptance of living with the disorder, however well managed, is often necessary.

Some sleep disorders have been found to compromise glucose metabolism.

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:

RBD is treatable. Medications are prescribed for RBD based on symptoms. Low doses of clonazepam is most effective with a 90% success rate. How this drug works to restore REM atonia is unclear: It is thought to suppress muscle activity, rather than directly restoring atonia. Melatonin is also effective and can also be prescribed as a more natural alternative. For those with Parkinson's and RBD, Levodopa is a popular choice. Pramipexole is another drug which can be an effective treatment option. Recent evidence has shown melatonin and clonazepam to be comparably effective in treatment of RBD with patients who received melatonin treatment reporting fewer side effects. In addition, patients with neurodegenerative diseases such as Parkinson's disease reported more favorable outcomes with melatonin treatment.

In addition to medication, it is wise to secure the sleeper's environment in preparation for episodes by removing potentially dangerous objects from the bedroom and either place a cushion round the bed or moving the mattress to the floor for added protection against injuries. Some extreme sufferers sleep in a sleeping bag zipped up to their neck, and wear mittens so they can't unzip it until they awake in the morning.

Patients are advised to maintain a normal sleep schedule, avoid sleep deprivation, and keep track of any sleepiness they may have. Treatment includes regulating neurologic symptoms and treating any other sleep disorders that might interfere with sleep. Sleep deprivation, alcohol, certain medications, and other sleep disorders can all increase RBD and should be avoided if possible.

Treatment of sleep apnea via a continuous positive airway pressure (CPAP) device has shown dramatic improvement in apnea and nearly complete resolution of RMD symptoms. Behavioral interventions may alleviate some RMD symptoms and movements. In such a therapy, sufferers are asked to perform RMD-like motions during the day in a slow and methodic manner. In such, patients come short of full rhythmic movements that they experience in sleep. Such behavioral training has been shown to carry over into sleep, and the forcefulness of the RMD movements is reduced or eliminated. Hypnosis and sleep restriction have been used in some cases to good effect.