Vibha M. Jha - Sleep: Evolution and Functions

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Mammalian species, including man, have almost exclusively provided the substrate for evaluating the intricate biological tapestry of sleep. Despite decades of intensive research, the functions and purpose of sleep remain elusive. By evaluating nonmammalian and mammalian organisms with a long evolutionary history may provide insight into our understanding of sleep and its functions. It may offer us some clues to find out the ways for sleep management and skills for our rapidly changing society, where the majority among us are living in a sleep-deficient condition.

The electrophysiological expression of sleep varies across phylogeny. In mammals and birds, there are two well-known, cyclically alternating electrophysiological patterns of sleep. They are characterized as non-rapid eye movement (NREM) sleep having high-amplitude and slow-wave electroencephalogram (EEG) activity with reduced muscle tone. Rapid eye movement (REM) sleep is characterized by a low amplitude (EEG) and rapid eye movements, almost similar to waking, but atonia of skeletal muscle is the hallmark of REM sleep. To date, all studied terrestrial mammals and birds exhibit these two distinct sleep stages. However, the same electrophysiology of mammalian NREM and REM sleep has not been convincingly demonstrated in invertebrates and lower vertebrates. Different electrophysiology in nonmammals, for example, distinctive high-amplitude spike activity in various reptiles, is correlated with behavioral sleep, which disappears in behavioral waking.

The imposition of mammalian sleep criteria on nonmammalian species has led to a significant, long-running controversy in the literature. Do the organisms other than mammals and birds have true sleep, or do they manifest significantly different unrelated correlates of sleep/rest? What are the phylogenetic origins of sleep? If this state has perpetuated in phylogeny throughout millions of years, then what does this reveal about the functions of sleep? Why have the phylogenetic and adaptive changes in sleep occurred during evolution? What are the adaptive changes occurring across the life-time, and why would it be? These are some questions that await answers.

Many varied functions have been proposed for sleep, including, among others, the development of neural circuitries, energy conservation, memory consolidation, synaptic plasticity, enhancement of the immune system, brain development, and many more. These functions have been proposed as a result of manipulations such as acute and chronic sleep deprivation, the effects of pharmacological agents, and genetic manipulations at the cellular level in animals model organisms. A current area of interest that ties these diverse topics together pertains to the understanding of human sleep disorders. In this book, we have attempted to explore the phylogenetic origins of sleep, functions of sleep, and the applicability of these findings to human sleep and treatment of its disorders.

The primary aims of this book are (1) to explore the evolution of sleep behavior from an evolutionary and phylogenetic perspective, (2) to evaluate the proposed functions of sleep utilizing experimental research in a variety of mammalian and nonmammalian species, and (3) to assess how this perspective can lead to a broader clinical understanding of human sleep management and its disorders.