According to a May 2014 article published in NewScientist, spectral analysis may help clinicians find objective evidence for sleep state misperception:

DSPD is diagnosed by a clinical interview, actigraphic monitoring, and/or a sleep diary kept by the patient for at least two weeks. When polysomnography is also used, it is primarily for the purpose of ruling out other disorders such as narcolepsy or sleep apnea. If a person can adjust to a normal daytime schedule on her/his own, with just the help of alarm clocks and will-power, the diagnosis is not given.

DSPD is frequently misdiagnosed or dismissed. It has been named as one of the sleep disorders most commonly misdiagnosed as a primary psychiatric disorder. DSPD is often confused with: psychophysiological insomnia; depression; psychiatric disorders such as schizophrenia, ADHD or ADD; other sleep disorders; or school refusal. Practitioners of sleep medicine point out the dismally low rate of accurate diagnosis of the disorder, and have often asked for better physician education on sleep disorders.

In medicine, insomnia is widely measured using the Athens insomnia scale. It is measured using eight different parameters related to sleep, finally represented as an overall scale which assesses an individual's sleep pattern.

A qualified sleep specialist should be consulted for the diagnosis of any sleep disorder so the appropriate measures can be taken. Past medical history and a physical examination need to be done to eliminate other conditions that could be the cause of insomnia. After all other conditions are ruled out a comprehensive sleep history should be taken. The sleep history should include sleep habits, medications (prescription and non-prescription), alcohol consumption, nicotine and caffeine intake, co-morbid illnesses, and sleep environment. A sleep diary can be used to keep track of the individual's sleep patterns. The diary should include time to bed, total sleep time, time to sleep onset, number of awakenings, use of medications, time of awakening and subjective feelings in the morning. The sleep diary can be replaced or validated by the use of out-patient actigraphy for a week or more, using a non-invasive device that measures movement.

Workers who complain of insomnia should not routinely have polysomnography to screen for sleep disorders. This test may be indicated for patients with symptoms in addition to insomnia, including sleep apnea, obesity, a thick neck diameter, or high-risk fullness of the flesh in the oropharynx. Usually, the test is not needed to make a diagnosis, and insomnia especially for working people can often be treated by changing a job schedule to make time for sufficient sleep and by improving sleep hygiene.

Some patients may need to do an overnight sleep study to determine if insomnia is present. Such a study will commonly involve assessment tools including a polysomnogram and the multiple sleep latency test. Specialists in sleep medicine are qualified to diagnose disorders within the, according to the ICSD, 81 major sleep disorder diagnostic categories. Patients with some disorders, including delayed sleep phase disorder, are often mis-diagnosed with primary insomnia; when a person has trouble getting to sleep and awakening at desired times, but has a normal sleep pattern once asleep, a circadian rhythm disorder is a likely cause.

In many cases, insomnia is co-morbid with another disease, side-effects from medications, or a psychological problem. Approximately half of all diagnosed insomnia is related to psychiatric disorders. In depression in many cases "insomnia should be regarded as a co-morbid condition, rather than as a secondary one;" insomnia typically predates psychiatric symptoms. "In fact, it is possible that insomnia represents a significant risk for the development of a subsequent psychiatric disorder." Insomnia occur in between 60% and 80% of people with depression. This may partly be due to treatment used for depression.

Determination of causation is not necessary for a diagnosis.

The disorder can be considered very likely in a totally blind person with periodic insomnia and daytime sleepiness, although other causes for these common symptoms need to be ruled out. In the research setting, the diagnosis can be confirmed, and the length of the free-running circadian cycle can be ascertained, by periodic assessment of circadian marker rhythms, such as the core body temperature rhythm, the timing of melatonin secretion, or by analyzing the pattern of the sleep–wake schedule using actigraphy. Most recent research has used serial measurements of melatonin metabolites in urine or melatonin concentrations in saliva. These assays are not currently available for routine clinical use.

Since 1979, the disorder has been recognized by the American Academy of Sleep Medicine:

- "Diagnostic Classification of Sleep and Arousal Disorders" (DCSAD), 1979: Non-24-Hour Sleep–Wake Syndrome; code C.2.d

- "The International Classification of Sleep Disorders", 1st & Revised eds. (ICSD), 1990, 1997: Non-24-Hour Sleep–Wake Syndrome (or Non-24-Hour Sleep–Wake Disorder); code 780.55-2

- "The International Classification of Sleep Disorders", 2nd ed. (ICSD-2), 2005: Non-24-Hour Sleep–Wake Syndrome (alternatively, Non-24-Hour Sleep–Wake Disorder); code 780.55-2

Since 2005, the disorder has been recognized by name in the U.S. National Center for Health Statistics and the U.S. Centers for Medicare and Medicaid Services in their adaptation and extension of the WHO's "International Statistical Classification of Diseases and Related Health Problems" (ICD):

- ICD-9-CM: Circadian rhythm sleep disorder, free-running type; code 327.34 became effective in October 2005. Prior to the introduction of this code, the nonspecific code 307.45, Circadian rhythm sleep disorder of nonorganic origin, was available, and as of 2014 remains the code recommended by the DSM-5.

- ICD-10-CM: Circadian rhythm sleep disorder, free running type; code G47.24 is due to take effect October 1, 2014.

Since 2013, the disorder has been recognized by the American Psychiatric Association:

- DSM-5, 2013: Circadian rhythm sleep–wake disorders, Non-24-hour sleep–wake type; ICD-9-CM code 307.45 is recommended (no acknowledgment of 327.34 is made), and ICD-10-CM code G47.24 is recommended when it goes into effect.

In general, there are two broad classes of treatment, and the two may be combined: psychological (cognitive-behavioral) and pharmacological. In situations of acute distress such as a grief reaction, pharmacologic measures may be most appropriate. With primary insomnia, however, initial efforts should be psychologically based, including discussion of good sleep hygiene. Other specific treatments are appropriate for some of the disorders, such as ingestion of the hormone melatonin, correctly timed bright light therapy and correctly timed dark therapy or light restriction for the circadian rhythm sleep disorders. Specialists in sleep medicine are trained to diagnose and treat these disorders, though many specialize in just some of them.

Middle-of-the-night insomnia is often treated with medication, although currently Intermezzo (zolpidem tartrate sublingual tablets) is the only Food and Drug Administration-approved medication specifically for treating MOTN awakening. Because most medications usually require 6–8 hours of sleep to avoid lingering effects the next day, these are often used every night at bedtime to prevent awakenings. Medication may not be prescribed in some cases, especially if the cause turns out to be the patient ingesting too much fluid during the day or just before they go to sleep.

Sleep restriction therapy and stimulus control therapy as described in insomnia have shown significance in treating middle of night insomnia.

Some studies have shown that zaleplon, which has a short elimination half-life, may be suitable for middle-of-the-night administration because it does not impair next day performance.

The light-dark cycle is the most important environmental time cue for entraining circadian rhythms of most species, including humans, and bright artificial light exposure has been developed as a method to improve circadian adaptation in night workers. The timing of bright light exposure is critical for its phase shifting effects. To maximize a delay of the body clock, bright light exposure should occur in the evening or first part of the night, and bright light should be avoided in the morning. Wearing dark goggles (avoiding bright light) or blue-blocking goggles during the morning commute home from work can improve circadian adaptation. For workers who want to use bright light therapy, appropriate fixtures of the type used to treat winter depression are readily available but patients need to be educated regarding their appropriate use, especially the issue of timing. Bright light treatment is not recommended for patients with light sensitivity or ocular disease.

A survey of 1.1 million residents in the United States found that those that reported sleeping about 7 hours per night had the lowest rates of mortality, whereas those that slept for fewer than 6 hours or more than 8 hours had higher mortality rates. Getting 8.5 or more hours of sleep per night was associated with a 15% higher mortality rate. Severe insomnia – sleeping less than 3.5 hours in women and 4.5 hours in men – is associated with a 15% increase in mortality.

With this technique, it is difficult to distinguish lack of sleep caused by a disorder which is also a cause of premature death, versus a disorder which causes a lack of sleep, and the lack of sleep causing premature death. Most of the increase in mortality from severe insomnia was discounted after controlling for co-morbid disorders. After controlling for sleep duration and insomnia, use of sleeping pills was also found to be associated with an increased mortality rate.

The lowest mortality was seen in individuals who slept between six and a half and seven and a half hours per night. Even sleeping only 4.5 hours per night is associated with very little increase in mortality. Thus, mild to moderate insomnia for most people is associated with increased longevity and severe insomnia is associated only with a very small effect on mortality. It is unclear why sleeping longer than 7.5 hours is associated with excess mortality.

What is considered objective insomnia, unlike SSM, can easily be confirmed empirically through clinical testing, such as by polysomnogram. Those who experience SSM may believe that they have not slept for extended periods of time, when they in fact do sleep but without perceiving it. For example, while patients who claim little or no sleep may usually acknowledge impaired job performance and daytime drowsiness, sleep state misperceivers often do not.

Cases of objective total insomnia are extremely rare. The few that have been recorded have predominantly been ascribed to a rare incurable genetic disorder called fatal familial insomnia, which patients rarely survive for more than 26 months after the onset of illness—often much less. While rarer cases of objective total insomnia lasting for decades have been reported, such as with the American Al Herpin and the Vietnamese Thai Ngoc, they have not been studied extensively in a clinical setting.

Polysomnograms can be used to help diagnose UARS. Patient who have UARS typically show multiple EEG arousals during the sleep study and little to no polygraphic evidence of obstructive sleep apnea or decreased levels of oxygen. UARS arousals, or respiratory-effort related arousals, typically last for one to three breaths. These arousals may be due to snoring, but patients do not need to snore in order to have UARS. Polysomnogram patterns must exhibit no evidence of apneas or hypopneas in order to be lead to a diagnosis of UARS. Even with polysomnography, diagnosis of UARS may be difficult because of insufficient means of measuring changes in airflow. This lack of sensitivity in detection may lead to misdiagnosis, as minor undetectable changes in airflow may still be responsible for the arousals. In order to definitively diagnose UARS, there must be a demonstrated pattern of greater negative esophageal pressures which are then followed by a rapid change to a more positive level with a sleep arousal. This can be confirmed with invasive polysomnography that uses an esophageal balloon transducer and full pneumotachograph.

Based on symptoms, patients are commonly misdiagnosed with chronic fatigue syndrome, fibromyalgia, or a psychiatric disorder such as ADHD or depression.

Melatonin is a hormone secreted by the pineal gland in darkness, normally at night. Its production is suppressed by light exposure, principally blue light around 460 to 480 nm. Light restriction, or dark therapy, in the hours before bedtime allows its production. Dark therapy does not require total darkness. Amber or orange colored goggles eliminate blue light to the eyes while allowing vision.

Melatonin is also available as an oral supplement. In the US and Canada, the hormone melatonin is not classified as a drug; it is sold as a dietary supplement. In other countries it requires a prescription or is unavailable. Although it is not licensed by the FDA as a treatment for any disorder, there have been no serious side effects or complications reported to date.

Melatonin has been shown to accelerate the adaptation of the circadian system to a nighttime work schedule. Melatonin may benefit daytime sleep in night workers by an additional direct sleep promoting mechanism. Melatonin treatment may increase sleep length during both daytime and nighttime sleep in night shift workers.

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.

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.

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.

Due to rapidly increasing knowledge about sleep in the 20th century, including the discovery of REM sleep in the 1950s and circadian rhythm disorders in the 70s and 80s, the medical importance of sleep was recognized. The medical community began paying more attention than previously to primary sleep disorders, such as sleep apnea, as well as the role and quality of sleep in other conditions. By the 1970s in the USA, clinics and laboratories devoted to the study of sleep and sleep disorders had been founded, and a need for standards arose.

Specialists in Sleep Medicine were originally certified by the American Board of Sleep Medicine, which still recognizes specialists. Those passing the Sleep Medicine Specialty Exam received the designation "diplomate of the ABSM." Sleep Medicine is now a recognized subspecialty within internal medicine, family medicine, pediatrics, otolaryngology, psychiatry and neurology in the United States. Certification in Sleep Medicine shows that the specialist:"has demonstrated expertise in the diagnosis and management of clinical conditions that occur during sleep, that disturb sleep, or that are affected by disturbances in the wake-sleep cycle. This specialist is skilled in the analysis and interpretation of comprehensive polysomnography, and well-versed in emerging research and management of a sleep laboratory."

Competence in sleep medicine requires an understanding of a myriad of very diverse disorders, many of which present with similar symptoms such as excessive daytime sleepiness, which, in the absence of volitional sleep deprivation, "is almost inevitably caused by an identifiable and treatable sleep disorder", such as sleep apnea, narcolepsy, idiopathic hypersomnia, Kleine–Levin syndrome, menstrual-related hypersomnia, idiopathic recurrent stupor, or circadian rhythm disturbances. Another common complaint is insomnia, a set of symptoms which can have a great many different causes, physical and mental. Management in the varying situations differs greatly and cannot be undertaken without a correct diagnosis.

Sleep dentistry (bruxism, snoring and sleep apnea), while not recognized as one of the nine dental specialties, qualifies for board-certification by the American Board of Dental Sleep Medicine (ABDSM). The resulting Diplomate status is recognized by the American Academy of Sleep Medicine (AASM), and these dentists are organized in the Academy of Dental Sleep Medicine (USA). The qualified dentists collaborate with sleep physicians at accredited sleep centers and can provide oral appliance therapy and upper airway surgery to treat or manage sleep-related breathing disorders.

In the UK, knowledge of sleep medicine and possibilities for diagnosis and treatment seem to lag. Guardian.co.uk quotes the director of the Imperial College Healthcare Sleep Centre: "One problem is that there has been relatively little training in sleep medicine in this country – certainly there is no structured training for sleep physicians." The Imperial College Healthcare site shows attention to obstructive sleep apnea syndrome (OSA) and very few other sleep disorders. Some NHS trusts have specialist clinics for respiratory and/or neurological sleep medicine.

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.

Possible treatments for circadian rhythm sleep disorders include:

- Behavior therapy or advice about sleep hygiene where the patient is told to avoid naps, caffeine, and other stimulants. They are also told to not be in bed for anything besides sleep and sex.

- Dark therapy, for example the use of blue-blocking goggles, is used to block blue- and bluegreen wavelength light from reaching the eye during evening hours so that the production of melatonin is not decreased or eliminated.

- Medications such as melatonin and modafinil (Provigil), or other short term sleep aids or wake-promoting agents can be beneficial; the former is a natural neurohormone responsible partly and in tiny amounts for the human body clock. The melatonin agonist Tasimelteon, trade name Hetlioz, has been approved in the USA solely for the treatment of non-24-hour sleep–wake disorder in totally blind people.

- Sleep phase chronotherapy may progressively advance or delay sleep time.

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.

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.

"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.

Behavioral modifications include getting at least 7–8 hours of sleep and lifestyle changes to help weight loss to help reduce or eliminate symptoms. Positional therapy also has helped many patients ease their UARS symptoms. Sleeping on one's side rather than in a supine position or using positional pillows can provide relief, but these modifications may not be sufficient to treat more severe cases. Avoiding sedatives including alcohol and narcotics can help prevent the relaxation of airway muscles, and thereby reduce the chance of their collapse. Avoiding sedatives may also help to reduce snoring.

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.

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.

A study conducted by Nadja Olini, Salome Kurth, and Reto Huber assessed the relationship between cortical maturation and sleep, and further investigate how these parameters are affected by caffeine consumption. They assessed sleep and markers of maturation with electrophysiological recordings, and behavioral and structural readouts in juvenile rats. Their results showed slow wave activity similar to humans in that caffeine treatment exerted short-term stimulating effects and altered the trajectory of slow wave activity. Furthermore, mild caffeine affected sleep and resulted in alterations and maturational parameters. Ultimately, the study showed that caffeine consumption during a critical developmental period shows long-lasting effects on sleep and brain maturation.