The Evolution and Function of Sleep

Abstract

Sleep is a common physiological state appearing in the everyday life of humans and other animals. In humans, sleep occupies approximately one third of our whole lifetime. People have thus kept asking the question of why we sleep. Sleep deprivation in rats results in lethality, indicating its essential roles (Rechtschaffen A, Bergmann BM, Sleep 25:18–24, 2002; Rechtschaffen A, Bergmann BM, Everson CA, Kushida CA, Gilliland MA, Sleep 12:68–87, 1989). From the aspect of evolution, sleep or sleep-like states are conserved across diverse animal species, implying an existent function for fulfilling a common purpose that may benefit the survival of animals. Up to now, however, the function and mechanism of sleep are still largely unknown. Recently, simple genetic animal models including fruit flies (Drosophila melanogaster), roundworms (Caenorhabditis elegans), and zebrafish (Danio rerio) have been actively studied to reveal the evolutionarily conserved components of sleep, which may lead to solving the fundamental question about the evolutionary origin of sleep (Hendricks JC, Finn SM, Panckeri KA, Chavkin J, Williams JA, Sehgal A, Pack AI, Neuron 25:129–138, 2000; Raizen DM, Zimmerman JE, Maycock MH, Ta UD, You YJ, Sundaram MV, Pack AI, Nature 451:569–572, 2008; Shaw PJ, Cirelli C, Greenspan RJ, Tononi G, Science 287:1834–1837, 2000; Singh K, Ju JY, Walsh MB, DiIorio MA, Hart AC, Sleep 37:1439–1451, 2014; Zhdanova IV, Wang SY, Leclair OU, Danilova NP, Brain Res 903:263–268, 2001). In addition, with the development of new techniques such as two-photon microscopy, optogenetics, and pharmacogenetics, researchers have obtained more ability to observe and manipulate neurons or their activity. Partly owing to the breakthrough of such new tools, researchers have found some evidence suggesting that sleep serves several functions including memory consolidation, clearance of brain metabolites, spine remodeling, and brain development (Bushey D, Tononi G, Cirelli C, Science 332:1576–1581, 2011; Donlea JM, Thimgan MS, Suzuki Y, Gottschalk L, Shaw PJ, Science 332:1571–1576, 2011; Kayser MS, Yue Z, Sehgal A, Science 344:269–274, 2014; Rasch B, Buchel C, Gais S, Born J, Science 315:1426–1429, 2007; Rechtschaffen A, Bergmann BM, Everson CA, Kushida CA, Gilliland MA, Sleep 12:68–87, 1989; Xie L, Kang H, Xu Q, Chen MJ, Liao Y, Thiyagarajan M, O’Donnell J, Christensen DJ, Nicholson C, Iliff JJ, et al. Science 342:373–377, 2013; Yang G, Lai CS, Cichon J, Ma L, Li W, Gan WB, Science 344:1173–1178, 2014). These studies have shown the relationship between sleep and other biological processes in different animals, and it further brings us to the question of whether the function of sleep is only for one purpose or is for multiple purposes. Up to now, our knowledge about sleep seems to be merely the tip of the iceberg. Further research is needed to understand the general function of sleep across species.

Here, we first introduce general criteria for sleep, which allows its definition in animals other than mammals (Sect. 15.1). Then we introduce REM sleep and non-REM sleep, which are the two major sleep stages of mammalian and avian sleep (Sect. 15.2), and introduce studies and hypotheses related to how they evolved (Sect. 15.3). Next, we briefly introduce sleep in aquatic mammals, which have made a unique change from their ancestral mammals to adapt to their lifestyle (Sect. 15.4). Then we introduce the current progress in studies using simple genetic animal models, namely, zebrafish, fruit flies, and roundworms (Sect. 15.5 and Sect. 15.6). Finally, we compare the suggested functions of sleep between mammals and invertebrate animals (Sect. 15.7).