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.

Treatment, a set of management techniques, is specific to DSPD. It is different from treatment of insomnia, and recognizes the patients' ability to sleep well on their own schedules, while addressing the timing problem. Success, if any, may be partial; for example, a patient who normally awakens at noon may only attain a wake time of 10 or 10:30 with treatment and follow-up. Being consistent with the treatment is paramount.

Before starting DSPD treatment, patients are often asked to spend at least a week sleeping regularly, without napping, at the times when the patient is most comfortable. It is important for patients to start treatment well-rested.

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.

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.

A diagnosis of PTSD requires that the person has been exposed to an extreme stressor such as one that is life-threatening. Any stressor can result in a diagnosis of adjustment disorder and it is an appropriate diagnosis for a stressor and a symptom pattern that does not meet the criteria for PTSD, for example a partner being fired, or a spouse leaving. If any of the symptom pattern is present before the stressor, another diagnosis is required, such as brief psychotic disorder or major depressive disorder. Other differential diagnoses are schizophrenia or other disorders with psychotic features such as Psychotic disorders due to a general medical condition. Drug-induced psychotic disorders can be considered if substance abuse is involved.

The symptom pattern for acute stress disorder must occur and be resolved within four weeks of the trauma. If it lasts longer, and the symptom pattern fits that characteristic of PTSD, the diagnosis may be changed.

Obsessive compulsive disorder may be diagnosed for intrusive thoughts that are recurring but not related to a specific traumatic event.

A number of screening instruments are used for screening adults for PTSD, such as the Clinician-Administered PTSD Scale for "DSM-5" (CAPS-5), Primary Care PTSD Screen for "DSM-5" (PC-PTSD-5), PTSD Checklist for DSM-5 (PCL-5), and Dissociative Subtype of PTSD Scale (DSPS). The CAPS-5 is considered the gold-standard assessment recommended for use by the U.S. National Center for PTSD.

There are also several screening and assessment instruments for use with children and adolescents. These include the Child PTSD Symptom Scale (CPSS), Clinician-Administered PTSD Scale for "DSM-5" -Child/Adolescent version (CAPS-CA-5), Child Trauma Screening Questionnaire, and UCLA Posttraumatic Stress Disorder Reaction Index for "DSM-IV".

Certain children who are particularly attached to their mother or other family figure due to separation anxiety and/or attachment theory often suffer the onset early, in pre-school, crèche or before school starts.

School phobia is diagnosed primarily through questionnaires and interviews with doctors. Other methods like observation have not proven to be as useful. This is partly because (school) anxiety is an internal phenomenon. An example of a modern multidimensional questionnaire is the "Differential Power Anxiety Inventory 'approach, with twelve scales to diagnose four different areas: anxiety-inducing conditions, manifestations, coping strategies and stabilization forms."

- Cognitive and lifestyle exploration

- 'School Phobia Test' (SAT)

- 'Anxiety questionnaire for students', (AFS)

Approximately 1 to 5% of school-aged children have school refusal, though it is most common in 5- and 6-year olds and in 10- and 11-year olds, it occurs more frequently during major changes in a child’s life, such as entrance to kindergarten, changing from elementary to middle school, or changing from middle to high school. The problem may start following vacations, school holidays, summer vacation, or brief illness, after the child has been home for some time, and usually ends prior to vacations, school holidays, or summer vacation, before the child will be out of school for some time. School refusal can also occur after a stressful event, such as moving to a new house, or the death of a pet or relative.

The rate is similar within both genders, and although it is significantly more prevalent in some urban areas, there are no known socioeconomic differences.