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

Stylish lengths: Mate choice in flowers

The styles of flowers may represent an arena for pollen competition in the race to fertilize ovules. Accordingly, selection should favour a longer 'race' to better discriminate among variable pollen by increasing style length. Sampling across a taxonomically diverse range of wild and outcrossed species, we found that the distribution of style lengths within plants were skewed towards longer styles, as predicted. In self-pollinated domesticated species, where discrimination among pollen is less important, we found no such pattern. We conclude that style length is under directional selection towards longer styles as a mechanism for mate choice among pollen of variable quality

Patterns of Senescence in Human Cardiovascular Fitness: Vo2 Max in Subsistence and Industrialized Populations

Objectives—This study explores whether cardiovascular fitness levels and senescent decline are similar in the Tsimane of Bolivia and Canadians, as well as other subsistence and industrialized populations. Among Tsimane, we examine whether morbidity predicts lower levels and faster decline of cardiovascular fitness, or whether their lifestyle (e.g., high physical activity) promotes high levels and slow decline. Alternatively, high activity levels and morbidity might counterbalance such that Tsimane fitness levels and decline are similar to those in industrialized populations. Methods—Maximal oxygen uptake (VO2max) was estimated using a step test heart rate method for 701 participants. We compared these estimates to the Canadian Health Measures Survey and previous studies in industrialized and subsistence populations. We evaluated whether health indicators and proxies for market integration were associated with VO2max levels and rate of decline for the Tsimane. Results—The Tsimane have significantly higher levels of VO2max and slower rates of decline than Canadians; initial evidence suggests differences in VO2max levels between other subsistence and industrialized populations. Low hemoglobin predicts low VO2max for Tsimane women while helminth infection predicts high VO2max for Tsimane men, though results might be specific to the VO2max scaling parameter used. No variables tested interact with age to moderate decline. Conclusions—The Tsimane demonstrate higher levels of cardiovascular fitness than industrialized populations, but levels similar to other subsistence populations. The high VO2max of Tsimane is consistent with their high physical activity and few indicators of cardiovascular disease, measured in previous studies

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

Energetic Costs of Testosterone in Two Subsistence Populations

Objective Testosterone plays a role in mediating energetic trade-offs between growth, maintenance, and reproduction. Investments in a high testosterone phenotype trade-off against other functions, particularly survival-enhancing immune function and cellular repair; thus only individuals in good condition can maintain both a high testosterone phenotype and somatic maintenance. While these effects are observed in experimental manipulations, they are difficult to demonstrate in free-living animals, particularly in humans. We hypothesize that individuals with higher testosterone will have higher energetic expenditures than those with lower testosterone. Methods Total energetic expenditure (TEE) was quantified using doubly labeled water in n = 40 Tsimane forager-horticulturalists (50% male, 18–87 years) and n = 11 Hadza hunter-gatherers (100% male, 18–65 years), two populations living subsistence lifestyles, high levels of physical activity, and high infectious burden. Urinary testosterone, TEE, body composition, and physical activity were measured to assess potential physical and behavioral costs associated with a high testosterone phenotype. Results Endogenous male testosterone was significantly associated with energetic expenditure, controlling for fat free mass; a one standard deviation increase in testosterone is associated with the expenditure of an additional 96–240 calories per day. Discussion These results suggest that a high testosterone phenotype, while beneficial for male reproduction, is also energetically expensive and likely only possible to maintain in healthy males in robust condition

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