Physiology
DSPD is a disorder of the body's timing system—the biological clock. Individuals with DSPD might have an unusually long circadian cycle, might have a reduced response to the re-setting effect of daylight on the body clock and/or may respond overly to the delaying effects of evening light and too little to the advancing effect of light earlier in the day. In support of the increased sensitivity to evening light hypothesis, "the percentage of melatonin suppression by a bright light stimulus of 1,000 lux administered 2 hours prior to the melatonin peak has been reported to be greater in 15 DSPD patients than in 15 controls."
People with normal circadian systems can generally fall asleep quickly at night if they slept too little the night before. Falling asleep earlier will in turn automatically help to advance their circadian clocks due to decreased light exposure in the evening. In contrast, people with DSPD are unable to fall asleep before their usual sleep time, even if they are sleep-deprived. Sleep deprivation does not reset the circadian clock of DSPD patients, as it does with normal people.
People with the disorder who try to live on a normal schedule cannot fall asleep at a "reasonable" hour and have extreme difficulty waking because their biological clocks are not in phase with that schedule. Non-DSPD people who do not adjust well to working a night shift have similar symptoms (diagnosed as shift-work sleep disorder, SWSD).
In most cases, it is not known what causes the abnormality in the biological clocks of DSPD patients. DSPD tends to run in families, and a growing body of evidence suggests that the problem is associated with the hPer3 (human period 3) gene. There have been several documented cases of DSPD and non-24-hour sleep-wake syndrome developing after traumatic head injury.
There have been a few cases of DSPD developing into non-24-hour sleep-wake syndrome, a more severe and debilitating disorder in which the individual sleeps later each day. It has been suggested that, instead of (or perhaps in addition to) a reduced reaction to light in the morning, an abnormal over-sensitivity to light in the late evening might contribute to the odd non-circadian pattern.
Read more about this topic: Delayed Sleep Phase Disorder
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