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.

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.

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.

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

There have not been sufficient studies conducted to make conclusive statements about prevalence nor who tends to suffer EHS. One study found that 13.5% of a sample of undergrads reported at least one episode over the course of their lives, with higher rates in those also suffering from sleep paralysis.

As of 2014, no clinical trials had been conducted to determine what treatments are safe and effective; a few case reports had been published describing treatment of small numbers of people (two to twelve per report) with clomipramine, flunarizine, nifedipine, topiramate, carbamazepine, methylphenidate. Studies suggest that education and reassurance can reduce the frequency of EHS episodes. There is some evidence that individuals with EHS rarely report episodes to medical professionals.

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.

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.

For those patients who have not been able to stop this disorder on their own, doctors have been working to discover a treatment that will work for everyone. One treatment that Schenck and Mahowald studied consisted of psychotherapy combined with "environmental manipulation". This was usually done separately from the weight-reducing diets. However, during this study only 10 percent of the patients were able to lose more than one third of their initial excess weight, which was not a viable percentage. In addition, they reported that many of the patients experienced "major depression" and "severe anxiety" during the attempted treatments. This was not one of the most successful attempts to help those with NSRED.

However, Dr. R. Auger reported on another trial treatment where patients were treated utilizing pramipexole. Those conducting the treatment noticed how the nocturnal median motor activity was decreased, as was assessed by actigraphy, and individual progress of sleep quality was reported. Nevertheless, Augur also said, "27 percent of subjects had RLS (restless legs syndrome, a condition known to respond to this medication), and number and duration of waking episodes related to eating behaviors were unchanged." Encouraged by the positive response verified in the above-mentioned trial treatment, doctors and psychiatrists conducted a more recent study described by Auger as "efficacy of topiramate [an antiepileptic drug associated with weight loss] in 17 consecutive patients with NSRED." Out of the 65 percent of patients who continued to take the medication on a regular basis, all confirmed either considerable development or absolute remission of "night-eating" in addition to "significant weight loss" being achieved. This has been one of the most effective treatments discovered so far, but many patients still suffered from NSRED. Therefore, other treatments were sought after.

Such treatments include those targeted to associated sleep disorders with the hope that it would play an essential part of the treatment process of NSRED. In Schenck and Mahowald's series, combinations of cardibopa/L-dopa, codeine, and clonazepam were used to treat five patients with RLS and one patient with somnambulism and PLMS (periodic limb movements in sleep). These patients all were suffering from NSRED as well as these other disorders, and they all experienced a remission of their NSRED as a result of taking these drugs. Two patients with OSA (obstructive sleep apnea) and NSRED also reported as having a "resolution of their symptoms with nasal continuous positive airway pressure (nCPAP) therapy." Clonazepam monotherapy was also found to be successful in 50 percent of patients with simultaneous somnambulism. Interestingly, dopaminergic agents such as monotherapy were effective in 25 percent of the NSRED subgroup. Success with combinations of dopaminergic and opioid drugs, with the occasional addition of sedatives, also was found in seven patients without associated sleep disorders. In those for whom opioids and sedatives are relatively contraindicated (e.g., in those with histories of substance abuse), two case reports were described as meeting with success with a combination of bupropion, levodopa, and trazodone. Notably, hypnotherapy, psychotherapy, and various behavioral techniques, including environmental manipulation, were not effective on the majority of the patients studied. Nevertheless, Auger argue that behavioral strategies should complement the overall treatment plan and should include deliberate placement of food to avoid indiscriminate wandering, maintenance of a safe sleep environment, and education regarding proper sleep hygiene and stress management. Even with their extensive studies, Schenck and Mahowald did not find the success as Auger found by treating his patients with topiramate.

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.

EDS can be a symptom of a number of factors and disorders. Specialists in sleep medicine are trained to diagnose them. Some are:

- Insufficient quality or quantity of night time sleep.

- Misalignments of the body's circadian pacemaker with the environment (e.g. jet lag, shift work or other circadian rhythm sleep disorders).

- Another underlying sleep disorder, such as narcolepsy, sleep apnea, idiopathic hypersomnia or restless legs syndrome.

- Disorders such as clinical depression or atypical depression.

- Tumors, head trauma, anemia, kidney failure, hypothyroidism or an injury to the central nervous system.

- Drug abuse.

- Genetic predisposition

- Vitamin deficiency, such as Biotin deficiency

- Particular classes of prescription and OTC medication

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.

There are a number of management options for bedwetting. The following options apply when the bedwetting is not caused by a specifically identifiable medical condition such as a bladder abnormality or diabetes. Treatment is recommended when there is a specific medical condition such as bladder abnormalities, infection, or diabetes. It is also considered when bedwetting may harm the child's self-esteem or relationships with family/friends. Only a small percentage of bedwetting is caused by a specific medical condition, so most treatment is prompted by concern for the child's "emotional" welfare. Behavioral treatment of bedwetting overall tends to show increased self-esteem for children.

Parents become concerned much earlier than doctors. A study in 1980 asked parents and physicians the age that children should stay dry at night. The average parent response was 2.75 years old, while the average physician response was 5.13 years old.

Punishment is not effective and can interfere with treatment.

Once diagnosed, ASPD can be treated with bright light therapy in the evenings or behaviorally with chronotherapy. Unlike other sleep disorders, ASPD does not disrupt normal functioning at work during the day and the patient does not complain of excessive daytime sleepiness. If their ASPD is causing people to lose out on evening activities, including putting their own typical children to bed, they may be able to force themselves to stay up later than their circadian rhythm requires. A sufferer of ASPD will still wake up very early and if this cycle continues it can lead to chronic sleep deprivation and other sleep disorders.

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.).

The administration of immunotherapy, in association with chemotherapy or tumor removal, .

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.

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.

Thorough history regarding frequency of bedwetting, any period of dryness in between, associated daytime symptoms, constipation, and encopresis should be sought.

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

Breathing difficulties can occur, resulting from neuromyotonic activity of the laryngeal muscles. Laryngeal spasm possibly resulting from neuromyotonia has been described previously, and this highlights that, in patients with unexplained laryngospasm, neuromytonia should be added to the list of differential diagnoses.

Studies have shown subtly decreased metabolism on positron emission tomography (PET) and single photon emission computed tomography (SPECT) in the left inferior frontal and left temporal lobes. and or basal ganglia hypermetabolism. Ancillary laboratory tests including MRI and brain biopsy have confirmed temporal lobe involvement. Cranial MRI shows increased signal in the hippocampus.

Cerebral spinal fluid (CSF) shows normal protein, glucose, white blood cell, and IgG index but there are weak oligoclonal bands, absent in the blood. Marked changes in circadian serum levels of neurohormones and increased levels of peripheral neurotransmitters were also observed. The absence of morphological alterations of the brain pathology, the suggestion of diffusion of IgG into the thalamus and striatum, more marked than in the cortex (consistent with effects on the thalamolimbic system) the oligoclonal bands in the CSF and the amelioration after PE all strongly support an antibody-mediated basis for the condition. Raised CSF IgG concentrations and oligoclonal bands have been reported in patients with psychosis. Anti-acetylcholine receptors (anti-AChR) antibodies have also been detected in patients with thymoma, but without clinical manifestations of myasthenia gravis. There have also been reports of non-paraneoplastic limbic encephalitis associated with raised serum VGKC suggesting that these antibodies may give rise to a spectrum of neurological disease presenting with symptoms arising peripherally, centrally, or both. Yet, in two cases, oligoclonal bands were absent in the CSF and serum, and CSF immunoglobulin profiles were unremarkable.

RLS symptoms may gradually worsen with age, though more slowly for those with the idiopathic form of RLS than for patients who also have associated medical condition. Nevertheless, current therapies can control the disorder, minimizing symptoms and increasing periods of restful sleep. In addition, some patients have remissions, periods in which symptoms decrease or disappear for days, weeks, or months, although symptoms usually eventually reappear. Being diagnosed with RLS does not indicate or foreshadow another neurological disease.

An adult who is compelled to nap repeatedly during the day may have excessive daytime sleepiness; however, it is important to distinguish between occasional daytime sleepiness and excessive daytime sleepiness, which is chronic.

A number of tools for screening for EDS have been developed. One is the Epworth Sleepiness Scale which grades the results of a questionnaire. The ESS generates a numerical score from zero (0) to 24 where a score of ten [10] or higher may indicate that the person should consult a specialist in sleep medicine for further evaluation.

Another tool is the Multiple Sleep Latency Test (MSLT), which has been used since the 1970s. It is used to measure the time it takes from the start of a daytime nap period to the first signs of sleep, called sleep latency. The test is based on the idea that the sleepier people are, the faster they will fall asleep.

The Maintenance of Wakefulness Test (MWT) is also used to quantitatively assess daytime sleepiness. This test is performed in a sleep diagnostic center. The test is similar to the MSLT. However, during this test the patient is instructed to try to stay awake.

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