Sleep as an evolved behavior: ecological opportunity costs and sleep optimization

Sleep problems afflict millions world-over. Treating this has been difficult because there is no consensus definition for “normal” sleep. People can vary in their personal sleep need, but the determinants of variation in sleep duration are largely unknown, as is the criteria to determine how much variation is normal. Given that most diurnal mammals (including primates) appear to sleep from sunset to sunrise, the leading explanation for sleep pathology in the post-industrial world has been that electronics, especially light illuminating devices, substantially reduce sleep duration. This assertion has heretofore only been tested experimentally. This research aims to resolve this issue by testing a proposed tradeoff model of sleep duration: individuals can decide when and how much time they want to invest in sleep versus other types of activity, depending on the relative costs and benefits to each. The specific alternative behaviors hypothesized in this dissertation are that sleep inhibition becomes adaptive when 1) food scarcity creates a sufficiently high opportunity cost for nighttime food acquisition and 2) dangerous environments incentivize maintenance of higher nighttime vigilance. To test these hypotheses in an evolutionarily informative ecology, research was conducted within small-scale subsistence (humans) populations: Hadza of Tanzania, San of Namibia, and primarily, Tsimane in Bolivia. Using the validated method of sleep accelerometry, sleep was recorded in free living conditions. To supplement this data, structured interviews were designed to measure nighttime behavioral profile, nighttime hunger, and sleep interruption. In all three study populations, sleep durations (5.-7.1h / night) closely resembled one another and even the typical rates observed in post-industrial societies. Among Tsimane, during periods of food scarcity, nighttime food production is significantly more common, but associated with severely shortened sleep duration. Tsimane also wake during the night due to unpredictable causes, especially when they sleep in houses lacking walls. Analyses suggest that these people may be going to be earlier and spending longer in bed to buffer total sleep against any possible sleep interruptions. Together, I hope that this research can create a foundation for studying sleep as a highly flexible phenotype (behavior), optimized against important but highly variable alternatives

Sleep Variability and Nighttime Activity among Tsimane Forager‐Horticulturalists

Objectives A common presumption in sleep research is that “normal” human sleep should show high night‐to‐night consistency. Yet, intra‐individual sleep variation in small‐scale subsistence societies has never been studied to test this idea. In this study, we assessed the degree of nightly variation in sleep patterns among Tsimane forager‐horticulturalists in Bolivia, and explored possible drivers of the intra‐individual variability. Methods We actigraphically recorded sleep among 120 Tsimane adults (67 female), aged 18–91, for an average of 4.9 nights per person using the Actigraph GT3X and Philips Respironics Actiwatch 2. We assessed intra‐individual variation using intra‐class correlations and average deviation from each individual\u27s average sleep duration, onset, and offset times ( ). Results Only 31% of total variation in sleep duration was due to differences among different individuals, with the remaining 69% due to nightly differences within the same individuals. We found no statistically significant differences in Tsimane sleep duration by day‐of‐the‐week. Nightly variation in sleep duration was driven by highly variable sleep onset, especially for men. Nighttime activities associated with later sleep onset included hunting, fishing, housework, and watching TV. Conclusions In contrast to nightly sleep variation in the United States being driven primarily by “sleeping‐in” on weekends, Tsimane sleep variation, while comparable to that observed in the United States, was driven by changing “bedtimes,” independent of day‐of‐the‐week. We propose that this variation may reflect adaptive responses to changing opportunity costs to sleep/nighttime activity

Methodological Differences Cannot Explain Associations Between Health, Anthropometrics, and Excess Resting Metabolic Rate

We appreciate Ocobock\u27s interest in methodological rigor. We largely agree with her commentary, which suggests that departures from standard protocols might have contributed to the high resting metabolic rate (RMR) measured for Tsimane. Indeed, our paper acknowledges many of the key departures from gold-standard indirect calorimetry methods of RMR assessment and attempts to adjust for several of these (Gurven et al., 2016). Bringing standard clinical methods into remote field settings often involves certain compromises, especially in our case, where RMR measurement was just one component of a large-scale health and aging project (Gurven et al., 2017). RMR data collection was from 2012 to 2014, and where we to measure RMR again for focused follow up, we would consider new available technologies, improve our protocol to the extent possible, and compare against our published estimates

Natural Sleep and Its Seasonal Variations in Three Pre-Industrial Societies

How did humans sleep before the modern era? Because the tools to measure sleep under natural conditions were developed long after the invention of the electric devices suspected of delaying and reducing sleep, we investigated sleep in three preindustrial societies[1–3]. We find that all three show similar sleep organization, suggesting that they express core human sleep patterns, likely characteristic of pre-modern era Homo sapiens. Sleep periods, the times from onset to offset, averaged 6.9–8.5-h, with sleep durations of 5.7–7.1-h, amounts near the low end of those industrial societies[4–7]. There was a difference of nearly 1-h between summer and winter sleep. Daily variation in sleep duration was strongly linked to time of onset, rather than offset. None of these groups began sleep near sunset, onset occurring, on average, 3.3-h after sunset. Awakening was usually before sunrise. The sleep period consistently occurred during the nighttime period of falling environmental temperature, was not interrupted by extended periods of waking and terminated, with vasoconstriction, near the nadir of daily ambient temperature. The daily cycle of temperature change, largely eliminated from modern sleep environments, may be a potent natural regulator of sleep. Light exposure, was maximal in the morning greatly decreasing at noon, indicating that all three groups seek shade at midday and that light activation of the suprachiasmatic nucleus is maximal in the morning. Napping occurred on \u3c7% of days in winter and \u3c22% of days in summer. Mimicking aspects of the natural environment might be effective in treating certain modern sleep disorders

Human genetics and sleep behavior

Why we sleep remains one of the greatest mysteries in science. In the past few years, great advances have been made to better understand this phenomenon. Human genetics has contributed significantly to this movement, as many features of sleep have been found to be heritable. Discoveries about these genetic variations that affect human sleep will aid us in understanding the underlying mechanism of sleep. Here we summarize recent discoveries about the genetic variations affecting the timing of sleep, duration of sleep and EEG patterns. To conclude, we also discuss some of the sleep-related neurological disorders such as Autism Spectrum Disorder (ASD) and Alzheimer's Disease (AD) and the potential challenges and future directions of human genetics in sleep research

Circadian Rhythm and Sleep Disruption: Causes, Metabolic Consequences and Countermeasures.

Circadian (∼ 24 hour) timing systems pervade all kingdoms of life, and temporally optimize behaviour and physiology in humans. Relatively recent changes to our environments, such as the introduction of artificial lighting, can disorganize the circadian system, from the level of the molecular clocks that regulate the timing of cellular activities to the level of synchronization between our daily cycles of behaviour and the solar day. Sleep/wake cycles are intertwined with the circadian system, and global trends indicate that these too are increasingly subject to disruption. A large proportion of the world's population is at increased risk of environmentally-driven circadian rhythm and sleep disruption, and a minority of individuals are also genetically predisposed to circadian misalignment and sleep disorders. The consequences of disruption to the circadian system and sleep are profound and include myriad metabolic ramifications, some of which may be compounded by adverse effects on dietary choices. If not addressed, the deleterious effects of such disruption will continue to cause widespread health problems; therefore, implementation of the numerous behavioural and pharmaceutical interventions that can help restore circadian system alignment and enhance sleep will be important

Sleep and nesting behavior in primates: A review

Sleep is a universal behavior in vertebrate and invertebrate animals, suggesting it originated in the very first life forms. Given the vital function of sleep, sleeping patterns and sleep architecture follow dynamic and adaptive processes reflecting trade‐offs to different selective pressures. Here, we review responses in sleep and sleep‐related behavior to environmental constraints across primate species, focusing on the role of great ape nest building in hominid evolution. We summarize and synthesize major hypotheses explaining the proximate and ultimate functions of great ape nest building across all species and subspecies; we draw on 46 original studies published between 2000 and 2017. In addition, we integrate the most recent data brought together by researchers from a complementary range of disciplines in the frame of the symposium “Burning the midnight oil” held at the 26th Congress of the International Primatological Society, Chicago, August 2016, as well as some additional contributors, each of which is included as a “stand‐alone” article in this “Primate Sleep” symposium set. In doing so, we present crucial factors to be considered in describing scenarios of human sleep evolution: (a) the implications of nest construction for sleep quality and cognition; (b) the tree‐to‐ground transition in early hominids; (c) the peculiarities of human sleep. We propose bridging disciplines such as neurobiology, endocrinology, medicine, and evolutionary ecology, so that future research may disentangle the major functions of sleep in human and nonhuman primates, namely its role in energy allocation, health, and cognition