Why do we and other animals sleep? When we are asleep, we are not performing activities that are important for reproductive success, such as locating food, caring for offspring, or finding mates. In the wild, sleep might make an animal more vulnerable to predation, and it certainly interferes with vigilance for predators. Sleep is found across the animal kingdom, yet it varies remarkably in its most fundamental characteristics across species. And for almost every pattern associated with sleep, exceptions can be found. For all of these reasons, sleep continues to be an evolutionary puzzle. Fortunately, sleep also has attracted much scientific interest, with many significant findings in the past 10 years.

The aim of this volume is to summarize recent advances in our understanding of the diversity of sleep patterns found in animals. Many of the chapters that follow examine sleep in different taxonomic groups, including insects, fish, reptiles, birds, and mammals. We take this “comparative approach” because it is one of the key ways in which biologists investigate the evolution of a trait (Harvey & Pagel, 1991). Indeed, the comparative method has long been used to investigate the evolution of sleep, particularly in mammals (e.g., Meddis, 1983; Zepelin, 1989). More recent comparative studies have capitalized on advances in the study of phylogenetic relationships to test hypotheses on the evolution of sleep (Capellini, Barton, Preston, et al., 2008a; Lesku, Roth, Amlaner, et al., 2006; Preston, Capellini, McNamara, et al., 2009; Roth, Lesku, Amlaner, et al., 2006).