As a major androgen with a key role in developing and maintaining male sexual characteristics, testosterone exerts a broad range of actions regulating several systems throughout human life. This article reviews data on the role of sleep in modulating endocrine release, as well as data on the effects of testosterone and androgen-replacement therapy on sleep architecture and breathing. This review also discusses the interaction of testosterone and sleep, with a particular emphasis on bilateral effects. Changes in nocturnal testosterone are sleep-related, with levels rising during sleep and falling on waking, whereas circadian effects are apparently marginal. Peak testosterone levels coincide with rapid-eye movement (REM) sleep onset. The decreasing sleep efficiency and numbers of REM sleep episodes with altered REM sleep latency observed in older men are associated with lower concentrations of circulating testosterone. The well-established male preponderance of sleep apnea suggests that sex hormones are involved in the pathogenesis of this breathing disorder. In addition, androgens are most certainly key players in the central and peripheral modulation of erectile function. Among other effects, sleep curtailment has been shown to lead to reduced levels of circulating androgens in healthy men and male rodents, and this highlights the biological significance of sleep homeostasis for endocrine regulation.