To sleep or not to sleep: the ecology of sleep in artificial organisms

The Complex Relationship between Sleep and Cognitive Reserve: A Narrative Review Based on Human Studies

Sleep and brain/cognitive/neural reserve significantly impact well-being and cognition throughout life. This review aims to explore the intricate relationship between such factors, with reference to their effects on human cognitive functions. The specific goal is to understand the bidirectional influence that sleep and reserve exert on each other. Up to 6 February 2024, a methodical search of the literature was conducted using the PubMed database with terms related to brain, cognitive or neural reserve, and healthy or disturbed sleep. Based on the inclusion criteria, 11 articles were selected and analyzed for this review. The articles focus almost exclusively on cognitive reserve, with no explicit connection between sleep and brain or neural reserve. The results evidence sleep's role as a builder of cognitive reserve and cognitive reserve's role as a moderator in the effects of physiological and pathological sleep on cognitive functions. In conclusion, the findings of the present review support the notion that both sleep and cognitive reserve are critical factors in cognitive functioning. Deepening comprehension of the interactions between them is essential for devising strategies to enhance brain health and resilience against age- and pathology-related conditions.

Variation in resting strategies across trophic levels and habitats in mammals

Mammals must carefully balance rest with other behaviors that influence fitness (e.g., foraging, finding a mate) while minimizing predation risk. However, factors influencing resting strategies and the degree to which resting strategies are driven by the activities of predators and/or prey remain largely unknown. Our goal was to examine how mammalian resting strategies varied with trophic level, body mass, and habitat. We reviewed findings from 127 publications and classified the resting strategies of terrestrial and aquatic mammalian species into three categories: social (e.g., resting in groups), temporal (e.g., resting during the day), or spatial (e.g., resting in burrows). Temporal strategies were most common (54% of cases), but the prevalence of strategies varied with body mass and among trophic levels. Specifically, lower trophic levels and smaller species such as rodents and lagomorphs used more spatial and social resting strategies, whereas top predators and larger species used mostly temporal resting strategies. Resting strategies also varied among habitat types (e.g., rainforest vs. grassland), but this was primarily because closely related species shared both habitats and resting strategies. Human presence also affected resting strategies at all trophic levels but most strongly influenced top predators through shifts in rest timing. Human-induced behavioral changes in rest patterns cascade to modify behaviors across multiple trophic levels. These findings advance our fundamental understanding of natural history and ecology in wild animals and provide a roadmap for future comparative studies.

Neuroendocrine Control of Sleep

Sleep is a phenomenon in animal behavior as enigmatic as it is ubiquitous, and one deeply tied to endocrine function. Though there are still many unanswered questions about the neurochemical basis of sleep and its functions, extensive interactions have been identified between sleep and the endocrine system, in both the endocrine system's effect on sleep and sleep's effect on the endocrine system. Unfortunately, until recent years, much research on sleep behavior largely disregarded its connections with the endocrine system. Use of both clinical studies and rodent models to investigate interactions between neuroendocrine function, including biological sex, and sleep therefore presents a promising area of further exploration. Further investigation of the neurobiological and neuroendocrine basis of sleep could have wide impact on a number of clinical and basic science fields. In this review, we summarize the state of basic sleep biology and its connections to the field of neuroendocrine biology, as well as suggest key future directions for the neuroendocrine regulation of sleep that may significantly impact new therapies for sleep disorders in women and men.

An Ecological Perspective on Sleep Disruption

Despite its evolutionary importance and apparent ubiquity among animals, the ecological significance of sleep is largely unresolved. The ecology of sleep has been particularly neglected in invertebrates. In insects, recent neurobehavioral research convincingly demonstrates that resting behavior shares several common characteristics with sleep in vertebrates. Laboratory studies have produced compelling evidence that sleep disruption can cause changes in insect daily activity patterns (via "sleep rebound") and have consequences for behavioral performance during active periods. However, factors that could cause insect sleep disruption in nature have not been considered nor have the ecological consequences. Drawing on evidence from laboratory studies, we argue that sleep disruption may be an overlooked component of insect ecology and could be caused by a variety of anthropogenic and nonanthropogenic factors in nature. We identify several candidate sleep-disrupting factors and provide new insights on the potential consequences of sleep disruption on individual fitness, species interactions, and ecosystem services. We propose an experimental framework to bridge the current gap in knowledge between laboratory and field studies. We conclude that sleep disruption is a potential mechanism underpinning variation in behavioral, population, and community-level processes associated with several aspects of global change.

Sleep as spatiotemporal integration of biological processes that evolved to periodically reinforce neurodynamic and metabolic homeostasis: The 2m3d paradigm of sleep

Sleep continues to perplex scientists and researchers. Despite decades of sleep research, we still lack a clear understanding of the biological functions and evolution of sleep. In this review, we will examine sleep from a functional and phylogenetic perspective and describe some important conceptual gaps in understanding sleep. Classical theories of the biology and evolution of sleep emphasize sensory activation, energy balance, and metabolic homeostasis. Advances in electrophysiology, functional neuroimaging, and neuroplasticity allow us to view sleep within the framework of neural dynamics. With this paradigm shift, we have come to realize the importance of neurodynamic homeostasis in shaping the biology of sleep. Evidently, animals sleep to achieve neurodynamic and metabolic homeostasis. We are not aware of any framework for understanding sleep where neurodynamic, metabolic, homeostatic, chronophasic, and afferent variables are all taken into account. This motivated us to propose the two-mode three-drive (2m3d) paradigm of sleep. In the 2m3d paradigm, local neurodynamic/metabolic (N/M) processes switch between two modes-m0 and m1-in response to three drives-afferent, chronophasic, and homeostatic. The spatiotemporal integration of local m0/m1 operations gives rise to the global states of sleep and wakefulness. As a framework of evolution, the 2m3d paradigm allows us to view sleep as a robust adaptive strategy that evolved so animals can periodically reinforce neurodynamic and metabolic homeostasis while remaining sensitive to their internal and external environment.

Ecology and neurophysiology of sleep in two wild sloth species

STUDY OBJECTIVES

Interspecific variation in sleep measured in captivity correlates with various physiological and environmental factors, including estimates of predation risk in the wild. However, it remains unclear whether prior comparative studies have been confounded by the captive recording environment. Herein we examine the effect of predation pressure on sleep in sloths living in the wild.

DESIGN

Comparison of two closely related sloth species, one exposed to predation and one free from predation.

SETTING

Panamanian mainland rainforest (predators present) and island mangrove (predators absent).

PARTICIPANTS

Mainland (Bradypus variegatus, five males and four females) and island (Bradypus pygmaeus, six males) sloths.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Electroencephalographic (EEG) and electromyographic (EMG) activity was recorded using a miniature data logger. Although both species spent between 9 and 10 h per day sleeping, the mainland sloths showed a preference for sleeping at night, whereas island sloths showed no preference for sleeping during the day or night. Standardized EEG activity during nonrapid eye movement (NREM) sleep showed lower low-frequency power, and increased spindle and higher frequency power in island sloths when compared to mainland sloths.

CONCLUSIONS

In sloths sleeping in the wild, predation pressure influenced the timing of sleep, but not the amount of time spent asleep. The preference for sleeping at night in mainland sloths may be a strategy to avoid detection by nocturnal cats. The pronounced differences in the NREM sleep EEG spectrum remain unexplained, but might be related to genetic or environmental factors.

Sleep and wake in rhythmic versus arrhythmic chronotypes of a microphthalmic species of African mole rat (Fukomys mechowii)

The giant Zambian mole rat (Fukomys mechowii) is a subterranean Afrotropical rodent noted for its regressed visual system and unusual patterns of circadian rhythmicity--within this species some individuals exhibit distinct regular circadian patterns of locomotor activity while others have arrhythmic circadian patterns. The current study was aimed at understanding whether differences in circadian chronotypes in this species affect the patterns and proportions of the different phases of the sleep-wake cycle. Physiological parameters of sleep (electroencephalogram and electromyogram) and behaviour (video recording) were recorded continuously for 72 h from 6 mole rats (3 rhythmic and 3 arrhythmic) using a telemetric system and a low-light CCTV camera connected to a DVD recorder. The results indicate that the arrhythmic individuals spend more time in waking with a longer average duration of a waking episode, less time in non-rapid eye movement (NREM) with a shorter average duration of an NREM episode though a greater NREM sleep intensity, and similar sleep cycle lengths. The time spent in rapid eye movement (REM) and the average duration of an REM episode were similar between the chronotypes.