The behavioral ecology of sleep: Examining the influence of exogenous and endogenous factors on sleep modification in the house sparrow, Passer domesticus

The sleep-wake cycle is perhaps the most important biological rhythm in the animal kingdom, yet the function of sleep remains elusive. To date, most sleep research has been conducted in the laboratory on mammalian species. However, the artificial laboratory environment and the use of restrictive electrophysiological equipment prevent the occurrence of "natural" sleep behavior, especially in small mammals and free-living species such as birds. In order to better answer functional questions about sleep, we need to expand the field of sleep research by including non-mammalian species and examining the adaptive nature of sleep in an ecological context to understand why this fundamental behavior evolved. I investigated the behavioral ecology of sleep in a wild bird species, the House Sparrow (Passer domesticus ), in a naturalistic setting using video-equipped nestboxes and continuously-recording radiotransmitters. I first validated that sleep behavior alone can be used as a non-invasive way to examine sleep, and established that we can make physiological inferences based on behavioral sleep characteristics. I next examined how sleep changes across the annual cycle in response to predictable changes in environment, behavior, and physiology, and explored these dynamics in light of various sleep theories proposed to date. Seasonal changes in sleep duration appear to be a function of nightlength, while seasonal changes in sleep continuity and daytime sleep appear to vary more strongly with behavior and physiology. Different aspects of sleep are thereby "maximized" in different seasons to most effectively restore homeostatic balance to the organism. I then examined how sleep changes within seasons in response to challenging daytime experiences and found that the sleep characteristics that are most prominent in a given season increase further. I also found that post-stress sleep modification can be status-dependent and vary by individual, so I then examined the role of individual reactivity to stress, and investigated corticosterone as a potential mediator of post-stress sleep changes. The sleep of individuals with high physiological reactivity was more affected by stress, because blocking corticosterone production resulted in greater increases in sleep duration and depth than in low-reactivity individuals. Corticosterone may therefore be involved in modulating certain post-stress sleep characteristics.