Unraveling the Evolutionary Determinants of Sleep

Abstract

Summary Despite decades of intense study, the functions of sleep are still shrouded in mystery. The difficulty in understanding these functions can be at least partly attributed to the varied manifestations of sleep in different animals. Daily sleep duration can range from 4–20 hrs among mammals, and sleep can manifest throughout the brain, or it can alternate over time between cerebral hemispheres, depending on the species. Ecological factors are likely to have shaped these and other sleep behaviors during evolution by altering the properties of conserved arousal circuits in the brain. Nonetheless, core functions of sleep are likely to have arisen early and to have persisted to the present day in diverse organisms. This review will discuss the evolutionary forces that may be responsible for phylogenetic differences in sleep and the potential core functions that sleep fulfills.

The Jellyfish Cassiopea Exhibits a Sleep-like State

“…The observation of behaviorally and mechanistically conserved sleep-like states across the animal kingdom [6,7] strongly supports the possibility for an early rooted sleep state rather than many instances of convergent evolution. It has been hypothesized that sleep has multiple functions, including synaptic homeostasis, regulation of neurotransmitters, repair of cellular damage, removal of toxins, memory consolidation and energy conservation [7], although the ancestral role and selective advantage of sleep remains elusive. Our discovery of a sleep-like state in an ancient metazoan phylum suggests that the ancestral role of sleep is rooted in basic requirements that are conserved across the animal kingdom.…”

“…To our knowledge, our finding is the first example of a sleep-like state in an organism with a diffuse nerve net [7,8], suggesting that this behavioral state arose prior to the evolution of a centralized nervous system. Though at least 600 million years of evolution separate cnidarians from bilaterians [9–16], many aspects of the nervous system are conserved, including neuropeptides and neurotransmitters [15–20].…”

“…Three behavioral characteristics define a sleep state [6,7,24]: (1) behavioral quiescence, a period of decreased activity; (2) reduced responsiveness to stimuli during the quiescent state; and (3) homeostatic regulation of the quiescent state. Both behavioral quiescence and reduced responsiveness must be rapidly reversible to differentiate sleep-like states from other immobile states (e.g.…”

“…In many animals sleep is regulated by both homeostatic and circadian systems [29], but this is not always the case [4–7,30]. For instance, the nematode C. elegans exhibits a developmentally regulated sleep state, and adult C. elegans show a non-circadian stress-induced-sleep state [4,5,31].…”

Sleep in the northern fur seal

“…Terrestrial mammals can display two distinctive sleep stages - slow wave sleep (SWS) and rapid eye movement (REM) sleep, which are usually scored based on EEG (electroencephalogram), EMG (electromyogram) and EOG (electrooculogram) features [1-4]. …”