Sleep is a complex behavioral state necessary for neurological, physical, cognitive, metabolic, and cardiovascular health throughout the lifespan. Poor sleep increases the risk of mortality among the general population. Sleep deprivation causes memory deficits, neuropsychiatric diseases, mood disorders, metabolic syndrome, cardiovascular diseases, stroke, and neurodegenerative diseases and carries an increased risk of suicide and motor vehicle accidents. Vitamin D is a fat-soluble secosteroid and its inadequacy is associated with multiple disorders, including bone health, neuropsychiatric diseases, metabolic disorders, and cardiovascular disorders. In recent years, vitamin D deficiency has been shown to affect sleep quality and many sleep disorders. Some studies have shown improvement in sleep disorders with vitamin D replacement, whereas others have not found any significant change. This chapter elaborates the role of vitamin D in different sleep disorders and describes the potential suggested mechanisms.