Light Therapy part 7

Overview of Light Therapy

Light Therapy & Sleep Disorders

Humans and animals generally have innate sleep-wake cycles close to but not exactly 24 hours. They depend on the daily light-dark cycle to keep their circadian rhythms to a regular 24 hours.
If a human is left in a room with no light-dark cues, he or she will gradually shift into a sleep-wake cycle that is not exactly 24 hours long. Body temperature and the secretion of the hormone melatonin follow the daily cycle. Other factors, such as work schedule can modify the sleep-wake cycle in humans. The autonomous cycle length varies at different periods in the life span. Adolescents often have an innate cycle longer than 24 hours so that they have the desire to stay up late and sleep in when it is time to get up. The innate cycle then shifts closer to 24 hours for adults, but for the elderly, the autonomous sleep-wake cycle may be shorter than 24 hours resulting in evening tiredness, sleep difficulty and waking too early. Individuals who have more severe difficulty with the timing of their sleep-wake cycle may have either Delayed Sleep Phase Disorder (difficulty falling sleep and the urge to sleep late) or Advanced Sleep Phase Disorder (tiring too early and waking too early). Both conditions can be treated with bright light (Terman et al, 1995).

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