ABSTRACT Sleep is a behavioral state whose quantity and quality represent a trade-off between the costs and benefits of sleep versus the costs and benefits of wakefulness. We humans need a brain/body state that can fulfill basic biological functions, such as securing food, water, and mates. But we must do so while conserving energy and minimizing exposure to dangers that accompany an organism’s particular ecological niche. Our species is particularly vulnerable during sleep because of our reduced ability to monitor the environment for nighttime predators and other environmental dangers. A number of variations in sleep characteristics may have evolved to reduce this vulnerability, both at the individual level and group level. The goals of this review paper are: (1) to explore such variations, including clinical disorders like insomnia and circadian rhythm disturbances, which may have adaptive utility in terms of enhancing detection of external threats, and (2) to consider cultural developments that improve vigilance and reduce vulnerability during sleep and the night.