Lemborexant is a dual orexin receptor antagonist, which inhibits orexin by binding competitively to two subtypes of orexin receptors. During normal periods of sleep, orexin system activity is suppressed, suggesting it is possible to purposefully facilitate the initiation and maintenance of sleep by interfering with orexin neurotransmission with lemborexant. Extensive in vitro and non-clinical testing of lemborexant supported the supposition that lemborexant has a low risk of QT prolongation at therapeutic and supratherapeutic exposures in humans. A Phase III study of lemborexant in insomnia is underway, and in addition, Eisai has announced the initiation of Phase II clinical studies of lemborexant in patients with irregular sleep-wake rhythm disorder.