This chapter discusses the influence of alterations in various endocrinological systems on sleep architecture, especially alterations that are associated with prolonged nocturnal sleep and/or daytime somnolence. Myxedema is associated with sleep apnea, and several mechanisms have been proposed to explain the association with hypothyroidism. Patients with hypothyroidism commonly present with disordered breathing. In congenital hypothyroidism, deficiency in interaction between the brain and thyroid hormones results in irreversible damage to the brain, presenting clinically as cretinism. Several studies examined the effects of exogenous testosterone on sleep-disordered breathing, and exogenous testosterone has been reported to increase sleep apnea in hypogonadal patients. Estrogen deficiency after menopause may cause central obesity, which is likely to be the main factor for the increased prevalence of sleep apnea in the post-menopausal period. While progesterone levels fall after menopause, there have been no data on the therapeutic effect of progesterone in sleep apnea.