Do We Sleep Better Than Our Ancestors? How Natural Selection and Modern Life Have Shaped Human Sleep

If you are lucky, you will spend approximately one-third of your life asleep, yet the reasons for sleep remain mysterious. Human sleep itself presents a paradox: we are the shortest sleeping primate, yet we have the largest brain. If sleep is for the brain, why do humans exhibit the least sleep? Could this be a result of natural selection shaping human sleep, or is it simply a negative consequence of modern lifestyles that reduce and disrupt sleep, with digital media, artificial light, and busy work schedules leading to changes in human sleep patterns? We suggest that it is a mixture of these two factors, but not all aspects of modern life have negative effects on sleep.

To investigate how sleep changed along the human lineage, we applied evolutionary methods to model the evolution of primate sleep, aiming to predict the human sleep phenotype as if humans were a typical primate¹. We discovered that humans sleep substantially less than expected, with the model predicting 9.5 hours, while the observed value is less than 7 hours of sleep per night (based on estimates in hunter-gatherers).^2,3 Within that shorter duration of sleep, humans pack a larger percentage of REM sleep than predicted, which is achieved by reducing non-REM sleep. Thus, our analyses inform how we reconcile a need for sleep with how much we sleep relative to other primates: humans appear to exhibit a sleep architecture that promotes sleep intensity, thus obtaining higher quality sleep within a shorter period of time.⁴

We hypothesize that reduced sleep occurred during transitions to terrestrial lifestyles and with increasing importance of social learning in human evolution. Nocturnal predators and threats from rival human groups would have made terrestrial sleep especially risky; simultaneously, sleep presents opportunity costs for learning, forming alliances, finding mates, and creating material objects. This radical shortening of sleep may help account for important health problems in humans; it may contribute, for example, to human susceptibility to Alzheimer’s disease, with fewer nighttime hours for neural maintenance during sleep.⁵

To better understand ancestral human sleep, we and other researchers have investigated non-electric, rural populations around the world as a proxy for sleep in our evolutionary ancestors.^3,6-8 We found that ancestral sleep based on these living proxies was likely to be short, highly fragmented, and substantially less refreshing than modern sleep in post-industrial populations. We also found evidence that circadian rhythms are more stable and less fragmented in these pre-industrial populations,⁸ yet individuals exhibit variation in chronotype, i.e., in the time going to bed and waking up,⁶ as we also see in post-industrial populations. Thus, in some respects, modern sleep has likely improved relative to ancestral sleep.

As industrialization progresses in a country, various factors shift sleep patterns, with some of these reducing the potential for maximally restorative sleep. Artificial lighting is one example: it provides important opportunities for education and pleasure, yet these activities cut into sleep durations.⁹ Many individuals also work at night, whether on assembly lines or pulling all-nighters to study, work, or travel internationally. The resulting alterations in physiological profiles contribute to increased health risks, including obesity, heart disease, diabetes, and even cancer.^10-12 Use of electronic devices, which emit “blue” light, also alter sleep patterns through effects on melatonin release,¹³ and we are only just beginning to understand how the proliferation of artificial lighting options impacts human physiology.¹⁴

Sleep environments have also changed, with positive and negative effects on sleep. Post-industrial populations sleep on more comfortable beds, and individuals in modern homes have more control over temperature, noise, and insects. In larger homes, individuals are also more likely to be sleeping alone, further reducing sleep disruptions, but tending to separate children from the close proximity and safety of their parents and contributing to childhood sleep disruptions.^15,16

Although we may feel separated from nature in post-industrial environments, the ecological drivers of sleep in mammals – such as predation risk¹⁷ – still play a role. In particular, psychosocial stress of various kinds can create a vigilance response, leading to difficulty falling and staying asleep.¹⁸ Large portions of post-industrial populations experience these psychosocial stressors through racial discrimination, immigration status, and economic hardship. Thus, the adaptive responses of sleep to stress may contribute to the marked health disparities that exist in modern populations, with altered sleep serving as one of the ways that the social and physical environment gets “under the skin” to influence health.¹⁹

In conclusion, we found that natural selection whittled away human sleep, driven by the risks and opportunity costs of sleep as our ancestors shifted to a more terrestrial lifestyle and social learning became more important for individual fitness. This may have generated tradeoffs that increased human susceptibility to late-life dementia,⁵ representing a mismatch between sleep durations and mechanisms for brain maintenance. Modern sleep presents additional forms of mismatch; some of these lifestyle factors enhance sleep by increasing its efficiency, while others interfere with our sleep, resulting in pervasive health disparities.

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