Main article: Circadian rhythm sleep disorder
Delayed sleep-phase syndrome (DSPS), also known as delayed sleep-phase disorder (DSPD) or delayed sleep-phase type (DSPT), is a circadian rhythm sleep disorder, a chronic disorder of the timing of sleep, peak period of alertness, core body temperature, hormonal and other daily rhythms. People with DSPS tend to fall asleep well after midnight and have difficulty waking up in the morning.
DSPS is a disorder of the body's timing system - the biological clock. Individuals with DSPS might have an unusually long circadian cycle, or might have a reduced response to the re-setting effect of light on the body clock.
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 advance their circadian clocks due to decreased light exposure in the evening. In contrast, people with DSPS are unable to fall asleep before their usual sleep time, even if they are sleep-deprived. Research has shown that sleep deprivation does not reset the circadian clock of DSPS patients, as it does with normal people.[10]
People with the disorder who try to live on a normal schedule have difficulty falling asleep and difficulty waking because their biological clocks are not in phase with that schedule. Normal people who do not adjust well to working a night shift have similar symptoms.
People with the disorder also show delays in other circadian markers, such as melatonin-secretion and the core body temperature minimum, that correspond to the delay in the sleep/wake cycle. The timing of sleepiness, spontaneous awakening, and these internal markers are all delayed by the same number of hours. Non-dipping blood pressure patterns are also associated with the disorder when present in conjunction with socially unacceptable sleeping and waking times.
In most cases, it is not known what causes the abnormality in the biological clocks of DSPS patients. DSPS tends to run in families,[11] and a growing body of evidence suggests that the problem is associated with the hPer3 (human period 3) gene.[12][13] There have been several documented cases of DSPS and non-24 hour sleep-wake syndrome developing after traumatic head injury.[14][15]
There have been a few cases of DSPS developing into non 24-hour sleep-wake syndrome, a more severe and debilitating disorder in which the individual sleeps later each day.[16]
In humans, melatonin is produced by the pineal gland, a gland about the size of a pea, located in the center of the brain. The melatonin signal forms part of the system that regulates the circadian cycle by chemically causing drowsiness and lowering the body temperature, but it is the central nervous system that controls the daily cycle in most components of the paracrine and endocrine systems[23][24] rather than the melatonin signal (as was once postulated).
