The timing of the sleep-wake cycle follows a circadian rhythm and this is under strict control by a set of genetic factors that are called "clock genes." The first mammalian clock gene, named Clock, was discovered in 1997. Since then multiple circadian clock genes have been described.
The core mammalian clock machinery consists of at least eight distinct proteins: BMAL1, CLOCK, PER1, PER2, PER3, CRY1, CRY2, and REV-ERB.
It is believed that malfunctions involving these proteins may lead to sleep disorders, especially circadian rhythm disorders. For example, mutations in PER2 are associated with familial advanced sleep phase syndrome. These clock genes are used throughout the body’s tissues. Their role in sleep disorders and other diseases is just beginning to be understood.
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Winrow, CJ et al. "Uncovering the genetic landscape for multiple sleep-wake traits." PLoS One 2009; 4(4):e5161.