In the Light of Evolution IV: The Human Condition
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This increase in body size led primates on many of these lineages to abandon fixed-point sleep sites, as naturally occurring enclosed sites would be challenging for larger animals to find. Similarly, the evolution of diurnal activity patterns—and associated shifts to living in larger groups [ 31 ]—would have made it even more difficult for larger groups of animals to locate fixed point sleep sites.
These factors led early primates to abandon the advantages of enclosed and sturdy sleep sites, and to instead sleep on tree branches.
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Sleeping on branches would have exposed these animals to increased risks from predation and to falling, especially because wind speeds, with punctuated gusts, are greater in the canopy [ 32 ]. Indeed, the primatology literature provides multiple accounts of primates falling from arboreal sleeping sites, resulting in injuries and death [ 33 , 34 ]. Great ape sleeping platforms show a conserved pattern of construction and function, and phylogenetic reconstruction points to emergence of this sleeping behavior sometime between 18 and 14 million years ago [ 38 ].
Typically, these platforms are built in trees that are selected for their firm, stable and resilient biomechanical properties [ 39—41 ]. Platforms are rebuilt each night, with each individual except dependent young building a separate sleeping nest. In sharp contrast, the lesser apes—the gibbons—do not build nests for sleeping. Instead, gibbons follow the pattern found in most monkeys: they typically sleep on branches in a lying or sitting position, with no environmental alterations [ 42 , 43 ]. Why do great apes build sleeping platforms?
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This is a simple reversal of cause and effect, where the cause is greater cognitive facility providing the opportunity to build effective sleep platforms, rather than use of platforms to enable greater cognitive performance. Recent captive research on apes has tested two crucial elements of the sleep quality hypothesis for the use of sleeping platforms in great apes. In a zoo study, Samson and Shumaker [ 46 ] provided orangutans with varied sleep materials, and then scored the quality of sleeping platforms the orangutans produced with different materials.
They found that sleeping platform quality was positively correlated with reduced arousability and lower sleep fragmentation i. In another study of zoo animals, Martin-Ordas and Call [ 47 ] found that, by making memory more resistance to the detrimental effects of interfering i. Increased body mass likely also played a role in the origins of great ape sleeping platforms [ 21 ]. In particular, larger-bodied great apes would find it more difficult to sleep on tree branches.
This effect would have favored individuals that built more resilient sleeping platforms to reduce the probability of lethal falls, and to reduce physical stressors on the bodies of sleeping individuals.
Once the use of sleeping platforms evolved, this could have enabled higher quality sleep within great apes, with emergent cognitive benefits. Human sleep has undergone additional changes from other great apes in several key features. An obvious feature is where we sleep, namely on the ground; among other apes, terrestrial sleep is rare, occurring only when predation risk is low, and typically only by very large bodied males [ 48—51 ]. In contrast, humans of both sexes habitually sleep on the ground, which could plausibly provide even more stable sleeping locations to achieve even deeper sleep.
Predation represents a major tradeoff in this context, with risk of predator attack thought to increase for terrestrial primates [ 52 , 53 ]. They suggested that when hominins became fully terrestrial they gained the advantage of greater stability than was possible in arboreal sleep. Freed from the disadvantages of arboreal sleep they could have achieved longer duration and higher quality sleep, which would have improved waking cognition.
Without terrestrial sleeping sites, they argue, fully human procedural memory consolidation for visual-motor skills and visual-spatial locations could not have evolved. The controlled use of fire may have been an essential precursor to secure ground sleep [ 20 ]. Arboreal sleeping platforms reduce predation risk [ 56 ] and minimize insect biting rates by masking host attractants or actually repelling insects [ 57 , 58 ]. Sleeping platforms also provide some insulation for warmth [ 57 ], and give a stable and secure environment to enable higher quality sleep [ 39 , 40 ].
A fire probably also reduces risk of predation and provides opportunities for thermoregulation, while smoke reduces insect activity [ 59 , 60 ].
Control of fire in early Homo erectus may therefore have enabled the night-time transition from trees to the ground [ 20 , 61 ]. Humans sleep the least compared to all other primates, yet have the greatest proportion of total sleep time dedicated to REM. As we discuss below when considering the potential evolutionary drivers of shorter sleep along the human lineage, tradeoffs between sleep and other activities are likely to be important factors.
When this same approach was applied to study the proportion of REM sleep in humans, the analyses revealed that humans pack a higher proportion of REM into their sleep than any other primate. It is worth noting, however, that some other primates have a longer absolute duration of REM sleep see Fig.
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As a last point of comparison to other primates, humans may be more flexible in the timing of sleep than our closest living relatives. Evidence from small-scale societies and subtropical hunter-gatherers [ 22 ], the historical record [ 64 ] and experiments in developed countries [ 65 ] suggest that humans show flexibility in their sleep. Flexibility can also occur in the context of daytime sleep, i. Counter to these findings and suggestions, however, a recent study of sleep in three hunter-gatherer populations [ 69 ] interpreted their actigraphy data as indicating consolidated sleep at night and with little napping during the day, and thus arguing against the flexibility of sleep.
This presents a challenge, and calls for better methods of assessing sleep phasing using actigraphy, including through use of new algorithms, validation with reported episodes of sleep and wakefulness, and development of new methods to better assess sleep without reliance on actigraphy.
It should be noted, however, that this study also revealed considerable heterogeneity in sleep onset time but less in awakening , consistent with flexibility in the timing of sleep. Given the global distribution of humans, adaptation to local conditions may be expected for sleep, as seen for other human phenotypes. One obvious aspect of this involves latitude, and the effects of large changes in day-length throughout the year. Unfortunately, however, sleep research in circumpolar environments has primarily focused on European populations [ 70 , 71 ] and the effects of latitude on the physiology of military personnel [ 72 ].
Thus, little is known regarding the effects of seasonally variable day—night cycles on the sleep-wake patterns of nonindustrial indigenous populations [ 12 ].
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Moreover, reports of sleep in post-industrial societies have shown conflicting evidence and small effects with respect to sleep duration across seasons [ 73 , 74 ]. Several factors may influence the outcome of such studies, including lack of direct exposure to changes in light and temperature among participants in laboratory environments, or the environmental buffer provided by modern work and residential facilities. In contrast, evidence supports the idea that sleep is modulated by season in traditional, equatorial societies; e.
Infant versus adult sleep. A sleep comparison between polyphasic human infant and consolidated sleep in an adult living in a post-industrial society adapted from reference [ 75 ]. Infant-parent co-sleeping has attracted much attention in recent decades, with parents faced with the dilemma of sleeping with the baby versus putting the baby in a separate room. All discussions of co-sleeping should begin by appreciating how radically novel it is for dependent children to even have the option to sleep separately from their caregivers.
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Throughout evolutionary history, families slept together, possibly with extended family members, and the same is true in many traditional societies today [ 59 , 77 ]. It is only in modern living conditions—with increased safety and availability of separate bedrooms for parents and children—that the dilemma of infant—parent co-sleeping arises. James McKenna was among the first anthropologists to investigate mother—infant night-time interactions empirically, often injecting an evolutionary perspective [ 78 , 79 ].
In some of this research, the investigators found that bed-sharing resulted in less deep sleep for mothers and infants, but more simultaneous awakenings by mothers and infants that were associated with more breastfeeding [ 24 ]. Overall, these studies demonstrate a mutually reinforcing relationship between mother—infant co-sleeping and feeding, probably reflecting correlated evolution among these behaviors. This research has been used to inform the potential risks associated with solitary sleep practices; e.
However, other studies have found that bed sharing also increases risk of SIDS, which may be amplified by factors such as infant age or use of alcohol or drugs [ 83 ].
The other insight to infant sleep comes in the context of infant crying, a feature not observed in chimpanzees [ 84 ]. Haig [ 23 ] revived and extended a hypothesis [ 85 ] that night-time arousal and crying by infants is an adaptive behavior to extend inter-birth intervals, benefiting the crying infant at the potential cost to parental reproductive success. Reviewing the literature, Haig [ 23 ] notes that shorter inter-birth intervals lead to greater offspring mortality, and that more night-time breastfeeding episodes results in longer postpartum amenorrhea.
Additionally, Haig [ 23 ] incorporated modern perspectives of genomic conflict by considering how imprinted genes of maternal origin might favor more consolidated sleep, whereas genes of paternal origin promote greater wakefulness. As noted by Haig [ 23 ], the explicit inter-generational and intra-genomic conflicts in his proposal challenge the assumption of mother—infant co-sleeping as a highly co-evolved and harmonious system that was suggested above in some of the research on co-sleeping.
To understand the reasons for short human sleep discussed above Fig. Or are ecological factors more informative of sleep durations, perhaps because they constrain how much time is available for sleep? This comparative perspective can help uncover the factors that have led humans to sleep so differently from other primates and perhaps more similarly to other mammals.
Although many earlier studies have investigated comparative patterns of sleep [ 86—88 ], here we focus on more recent studies that made use of larger sample sizes and improved statistical-phylogenetic methods [ 63 ]. Two independent research groups [ 89 , 90 ] have investigated the phylogenetic, ecological and life history drivers of sleep architecture, which is defined as the quantitative structure and pattern of sleep. We consider the major hypotheses for sleep duration that have been investigated comparatively, which fall into two broad categories: those in which ecological factors, such as diet, influence sleep durations; and other hypotheses proposing that specific functional benefits of sleep, such as memory consolidation, influence sleep architecture.
Among the ecological factors, several variables are considered to be important: i predation risk, with longer sleep times expected when animals have access to a safe and stable sleep site; ii metabolism, with higher metabolism either favoring more sleep to conserve energy, or less sleep to enable animals to better meet nutritional needs; and iii body mass or its correlates , with larger bodied animals needing more resources and thus having less time for sleep. In terms of functional benefits of sleep, one major hypothesis involves memory consolidation, with larger brained animals proposed to need more sleep [ 91 ].