In the natural world, the light/dark cycle entrains our rhythm. Light is a very effective “zeitgeber.” In the presence of environmental time cues, sleep–wake physiology and gene expression continue to exhibit a near-24-hour circadian rhythm regulated by the suprachiasmatic nucleus in the hypothalamus. Phototherapy is an accepted modality for the non-pharmacological management of circadian disorders, such as delayed sleep phase syndrome, insomnia, and mood disorders. This review discusses the clinical repercussions of circadian rhythm disorders, the physiological principle of phototherapy, and its application across the spectrum of sleep disorders.
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