<P> According to the International Classification of Sleep Disorders, Revised (ICSD - R, 2001), the circadian rhythm sleep disorders share a common underlying chronophysiologic basis: </P> <P> The major feature of these disorders is a misalignment between the patient's sleep - wake pattern and the pattern that is desired or regarded as the societal norm...In most circadian rhythm sleep disorders, the underlying problem is that the patient cannot sleep when sleep is desired, needed or expected . </P> <P> Incorporating minor updates (ICSD - 3, 2014), the diagnostic criteria for delayed sleep phase disorder are: </P> <Ol> <Li> An intractable delay in the phase of the major sleep period occurs in relation to the desired clock time, as evidenced by a chronic or recurrent (for at least three months) complaint of inability to fall asleep at a desired conventional clock time together with the inability to awaken at a desired and socially acceptable time . </Li> <Li> When not required to maintain a strict schedule, patients exhibit improved sleep quality and duration for their age and maintain a delayed phase of entrainment to local time . </Li> <Li> Patients have little or no reported difficulty in maintaining sleep once sleep has begun . </Li> <Li> Patients have a relatively severe to absolute inability to advance the sleep phase to earlier hours by enforcing conventional sleep and wake times . </Li> <Li> Sleep--wake logs and / or actigraphy monitoring for at least two weeks document a consistent habitual pattern of sleep onsets, usually later than 2 am, and lengthy sleeps . </Li> <Li> Occasional noncircadian days may occur (i.e., sleep is "skipped" for an entire day and night plus some portion of the following day), followed by a sleep period lasting 12 to 18 hours . </Li> <Li> The symptoms do not meet the criteria for any other sleep disorder causing inability to initiate sleep or excessive sleepiness . </Li> <Li> If one of the following laboratory methods is used, it must demonstrate a significant delay in the timing of the habitual sleep period: 1) 24 - hour polysomnographic monitoring (or two consecutive nights of polysomnography and an intervening multiple sleep latency test), 2) Continuous temperature monitoring showing that the time of the absolute temperature nadir is delayed into the second half of the habitual (delayed) sleep episode . </Li> </Ol>

Why can i not sleep at night but can during the day