Delayed sleep-phase disorder (DSPD), also known as Delayed sleep-phase syndrome (DSPS) or delayed sleep-phase type (DSPT), is a circadian rhythm sleep disorder, a chronic disorder of sleep timing. People with delayed sleep-phase disorder tend to fall asleep well after midnight and also have difficulty waking up in the morning.

Often, people with the disorder report that they cannot sleep until early morning, but they fall asleep at about the same time every "night", no matter what time they go to bed. Unless they have another sleep disorder such as sleep apnea in addition to delayed sleep-phase disorder, patients can sleep well, and have a normal need for sleep. Therefore, they find it very difficult to wake up in time for a typical school or work day if they have only slept for a few hours. However, they sleep soundly, wake up spontaneously, and do not feel sleepy again until their next "night" if they are allowed to follow their own late schedule, e.g. sleeping from 4 a.m. to noon.

The syndrome usually develops in early childhood or adolescence, and sometimes disappears in adolescence or early adulthood. It can be to a greater or lesser degree treatable, but cannot be cured.

Delayed sleep-phase disorder was first formally described in 1981 by Dr. Elliot D. Weitzman and others at Montefiore Medical Center. It is responsible for 7 -10% of cases of chronic insomnia. However, as few doctors are aware of its existence, it often goes untreated or is treated inappropriately. Delayed sleep-phase disorder is frequently misdiagnosed as primary insomnia or as a psychiatric condition.

Characteristics of DSPD

Sleep-onset and wake times that are intractably later than desired
Actual sleep-onset times at nearly the same daily clock hour
Little or no reported difficulty in maintaining sleep once sleep has begun
Extreme difficulty awakening at the desired time in the morning
A relatively severe to absolute inability to advance the sleep phase to earlier hours by enforcing conventional sleep and wake times.

The following features of delayed sleep-phase disorder distinguish it from other sleep disorders:

People with delayed sleep-phase disorder have at least a normal - and often much greater than normal - ability to sleep during the morning, and sometimes in the afternoon as well. In contrast, those with chronic insomnia do not find it much easier to sleep during the morning than at night.

People with delayed sleep-phase disorder fall asleep at more or less the same time every night, and sleep comes quite rapidly if the person goes to bed near the time he or she usually falls asleep. Young children with delayed sleep-phase disorder resist going to bed before they are sleepy, but the bedtime struggles disappear if they are allowed to stay up until the time they usually fall asleep.

Delayed sleep-phase disorder patients can sleep well and regularly when they can follow their own sleep schedule, e.g. on weekends and during vacations.

Delayed sleep-phase disorder is a chronic condition. Symptoms must have been present for at least one month before a diagnosis of delayed sleep-phase disorder can be made.

Attempting to force oneself through 9–5 life with delayed sleep-phase disorder has been compared to constantly living with 6 hours of jet lag. Often, sufferers manage only a few hours sleep a night during the working week, then compensate by sleeping until the afternoon on weekends. Sleeping in on weekends, and/or taking long naps during the day, gives people with the disorder relief from daytime sleepiness but also perpetuates the late sleep phase.

People with delayed sleep-phase disorder tend to be extreme night owls. They feel most alert and say they function best and are most creative in the evening and at night. delayed sleep-phase disorder patients cannot simply force themselves to sleep early. They may toss and turn for hours in bed, and sometimes not sleep at all, before reporting to work or school. Less extreme and more flexible night owls, and indeed morning larks, are within the normal chronotype spectrum.