Introduction: sleep and ecology
All mammals so far studied experience some form of sleep. When mammals are sleep-deprived, they generally attempt to regain the lost sleep by exhibiting a “sleep rebound,” suggesting that sleep serves important functions that cannot be neglected (Siegel, 2008; Zepelin, 1989; Zepelin, Siegel, & Tobler, 2005). When sleep deprivation is enforced on individuals, it is accompanied by impaired physiological functions and a deterioration of cognitive performance (Kushida, 2004; Rechtschaffen, 1998; Rechtschaffen & Bergmann, 2002). In the rat, prolonged sleep deprivation ultimately results in death (Kushida, 2004; Rechtschaffen & Bergmann, 2002). Together, these observations suggest that sleep is a fundamental requirement for mammalian life, and much research has focused on identifying the physiological benefits that sleep provides (Horne, 1988; Kushida, 2004).
Are there also costs associated with sleep? If so, what are the selective pressures that constrain the amount of time that individuals can devote to sleep? Sleep is probably associated with “opportunity costs” because sleeping animals cannot pursue other fitness-enhancing activities, such as locating food, maintaining social bonds, or finding mates. Sleeping animals may also pay direct costs. For example, sleep is a state of reduced consciousness, and thus sleeping individuals are less able to detect and escape from approaching predators (Allison & Cicchetti, 1976; Lima, Rattenborg, Lesku, et al., 2005). These ecological factors are likely to be important constraints on sleep durations and may also affect how sleep is organized over the daily cycle.