A quantitative, theoretical framework for understanding mammalian sleep

Abstract

Sleep is one of the most noticeable and widespread phenomena occurring in multicellular animals. Nevertheless, no consensus for a theory of its origins has emerged. In particular, no explicit, quantitative theory exists that elucidates or distinguishes between the myriad hypotheses proposed for sleep. Here, we develop a general, quantitative theory for mammalian sleep that relates many of its fundamental parameters to metabolic rate and body size. Several mechanisms suggested for the function of sleep can be placed in this framework, e.g., cellular repair of damage caused by metabolic processes as well as cortical reorganization to process sensory input. Our theory leads to predictions for sleep time, sleep cycle time, and rapid eye movement time as functions of body and brain mass, and it explains, for example, why mice sleep Ϸ14 hours per day relative to the 3.5 hours per day that elephants sleep. Data for 96 species of mammals, spanning six orders of magnitude in body size, are consistent with these predictions and provide strong evidence that time scales for sleep are set by the brain's, not the whole-body, metabolic rate.allometric scaling ͉ brain ͉ cellular repair ͉ metabolic rate ͉ sleep times I n contrast to other obvious physiological phenomena such as eating, breathing, and walking, neither the function nor the mechanism by which sleep occurs is well understood, despite its ubiquity (1). Indeed, the quest for a fundamental theory of sleep is considered one of the most important, unsolved problems in science (2-7). Recent neurobiological studies have made great advances in understanding the mechanisms involved in sleep. Hormones, cells, and enzymes whose levels of activity and expression vary during sleep and between sleeping and waking states have been identified (2,3,(8)(9)(10). Although these studies have been unable to determine the purpose of sleep, such investigations will play an increasingly important role in determining what processes are specific to sleep.Among the most studied and best known hypotheses for the purpose of sleep are the following: (i) rest for the body or brain and prevention from overheating (11-13); (ii) cortical reorganization and processing associated with memory and learning (14-17); and (iii) cellular repair in the body or brain (3,(18)(19)(20)(21)54). Some hypotheses for sleep are easy to eliminate. For example, hypothesis i has been rejected because the energy saved during sleep is minimal, amounting to approximately one frankfurter bun worth of calories per night for humans (11,12). Moreover, because rates of heating and cooling are set by mass-specific metabolic rate and body size, which should scale identically, equating the heat increase that leads to overheating with the heat lost to return to normal temperatures predicts that the ratio of sleep time to awake time is invariant with respect to body size. This is clearly counter to the observation that sleep times decrease with body size.Distinguishing among other theories of sleep (e.g., hypotheses i and ii), howeve...

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“…There are those who hypothesize that during sleep hidden connections are found between memories that are not apparent during the waking hours (Savage and West, 2007). However, there is no proof that the brain during sleep can access memories by a unique pathway not available to the brain during waking hours.…”

Two types of REM sleep: The atonic and brain REM sleep

“…Whereas, some mammals, such as horses and elephants (Savage & West, 2007;Williams et al, 2008), manifest REM sleep shorter in duration than carnivores (Savage & West, 2007) and pennipeds (Lyamin et al, 2002;Pryaslova et al, 2009), but longer than those in dolphins (Cai, 2015b;Lyamin et al, 2007;Madan & Jha, 2012;Sekiguchi et al, 2006;Siegel, 2008) and birds (Ayala-Guerrero et al, 1988Cai, 2015b). They can partly improve their muscular efficiency with atonic REM sleep, but may not be able to fully reduce their muscle tone.…”

“…The REM sleep functions in the species with long sleep duration Further longer in duration of REM sleep, in the mammals sleeping daily as long as carnivores (Savage & West, 2007) and pennipeds (Lyamin et al, 2002;Pryaslova et al, 2009), or even longer than they do, the effect of atonic REM sleep to improve the muscular efficiency should persist, notably for increasing the contractile strength in carnivores and pennipeds, but might become secondary as compared to other functions of REM sleep during the lengthy sleep. Especially, adaptive immobility (Meddis, 1975) is the most obvious function of REM sleep in the few mammals sleeping daily longer than carnivores and pennipeds, such as armadillo (Affanni et al, 2001), bat (Zhao et al, 2010), owl monkey (Sri Kantha et al, 2009;Suzuki & Sri Kantha, 2006), and so on.…”

“…Whereas, in most animals, the duration of REM sleep is inversely correlated with predation risk (Lesku et al, 2006;, so that immobility of REM sleep would bring disadvantage. Besides, it is also difficult to use adaptive immobility to account for the REM sleep lengthy in carnivores which need predominate from more predation (Savage & West, 2007), while short in birds which need take rest (Ayala-Guerrero et al, 1988;Cai, 2015b). Improvement of muscular efficiency is the more likely function of REM sleep for them (Cai, 2015b).…”

“…In this regard, the adaptive immobility in caves to avoid danger is actually rare as exceptions. Besides, it is also difficult to use adaptive immobility to account for the REM sleep lengthy in carnivores which need predominate from more predation (Savage & West, 2007) while short in birds which need take rest (AyalaGuerrero et al, 1988(AyalaGuerrero et al, , 2003Cai, 2015b). Whereas, as physiological studies suggested the REM sleep tend to d i s r u p t t h e e m o t i o n a l b a l a n c e t o w a r d depression (Goldstein & Walker, 2014;Hicks & Moore, 1979;Palagini et al, 2013;Vogel, 1975;1983), so that nor supported the proposal of Lesku et al which would predict the REM sleep to decrease fear (Lesku et al, 2006;.…”