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 recent 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 time4.
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 sleep5.
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 ancestors3, 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 populations8, yet individuals exhibit variation in chronotype, i.e., in the time going to bed and waking up6, 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 durations9. 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 cancer10-12. Use of electronic devices, which emit “blue” light, also alter sleep patterns through effects on melatonin release13, and we are only just beginning to understand how the proliferation of artificial lighting options impacts human physiology14.
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 disruptions15, 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 asleep18. 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 health19.
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 dementia5, 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.
Read the full Evolutionary Mismatch series:
- Introduction: Evolutionary Mismatch and What To Do About It by David Sloan Wilson
- Functional Frivolity: The Evolution and Development of the Human Brain Through Play by Aaron Blaisdell
- A Mother’s Mismatch: Why Cancer Has Deep Evolutionary Roots by Amy M. Boddy
- It’s Time To See the Light (Another Example of Evolutionary Mismatch) by Dan Pardi
- Generating Testable Hypotheses of Evolutionary Mismatch by Sudhindra Rao
- (Mis-) Communication in Medicine: A Preventive Way for Doctors to Preserve Effective Communication in Technologically-Evolved Healthcare Environments by Brent C. Pottenger
- The Darwinian Causes of Mental Illness by Eirik Garnas
- Is Cancer a Disease of Civilization? by Athena Aktipis
- The Potential Evolutionary Mismatches of Germicidal Ambient Lighting by Marcel Harmon
- Do We Sleep Better Than Our Ancestors? How Natural Selection and Modern Life Have Shaped Human Sleep by Charles Nunn and David Samson
- The Future of the Ancestral Health Movement by Hamilton M. Stapell
- Humans: Smart Enough to Create Processed Foods, Daft Enough to Eat Them by Ian Spreadbury
- Nunn, C.L., and Samson, D.R. (2018). Sleep in a Comparative Context: Investigating How Human Sleep Differs from Sleep in Other Primates. American Journal of Physical Anthropology, In Press.
- Samson, D.R., Crittenden, A.N., Mabulla, I.A., Mabulla, A.Z., and Nunn, C.L. (2017). Hadza sleep biology: Evidence for flexible sleep‐wake patterns in hunter‐gatherers. American Journal of Physical Anthropology 162(3), 573-582.
- Yetish, G., Kaplan, H., Gurven, M., Wood, B., Pontzer, H., Manger, P.R., Wilson, C., McGregor, R., and Siegel, J.M. (2015). Natural sleep and its seasonal variations in three pre-industrial societies. Current Biology 25, 2862-2868.
- Samson, D.R., and Nunn, C.L. (2015). Sleep intensity and the evolution of human cognition. Evolutionary Anthropology 24, 225-237.
- Nesse, R.M., Finch, C.E., and Nunn, C.L. (2017). Does selection for short sleep duration explain human vulnerability to Alzheimer’s disease? Evolution, Medicine, and Public Health 1, 39-46.
- Samson, D.R., Crittenden, A.N., Mabulla, I.A., Mabulla, A.Z., and Nunn, C.L. (2017). Chronotype variation drives night-time sentinel-like behaviour in hunter gatherers. Proceedings of the Royal Society B, In Press.
- Samson, D.R., Crittenden, A.N., Mabulla, I.A., and Mabulla, A.Z.P. (2017). The evolution of human sleep: Technological and cultural innovation associated with sleep-wake regulation among Hadza hunter-gatherers. Journal of Human Evolution 113, 91-102.
- Samson, D.R., Manus, M.B., Krystal, A.D., Fakir, E., Yu, J.J., and Nunn, C.L. (2017). Segmented sleep in a nonelectric, small‐scale agricultural society in Madagascar. American Journal of Human Biology e22979.
- de la Iglesia, H., Fernández-Duque, E., Golombek, D.A., Lanza, N., Duffy, J.F., Czeisler, C.A., and Valeggia, C.R. (2015). Access to electric light is associated with shorter sleep duration in a traditionally hunter-gatherer community. Journal of Biological Rhythms 30, 342-350.
- Gangwisch, J.E. (2014). A review of evidence for the link between sleep duration and hypertension. American Journal of Hypertension 27, 1235-1242.
- Chaput, J.-P., McNeil, J., Després, J.-P., Bouchard, C., and Tremblay, A. (2013). Short sleep duration as a risk factor for the development of the metabolic syndrome in adults. Preventive Medicine 57, 872-877.
- Haus, E.L., and Smolensky, M.H. (2013). Shift work and cancer risk: Potential mechanistic roles of circadian disruption, light at night, and sleep deprivation. Sleep Medicine Reviews 17, 273-284.
- Chang, A.-M., Aeschbach, D., Duffy, J.F., and Czeisler, C.A. (2015). Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proceedings of the National Academy of Sciences 112, 1232-1237.
- Zielinska-Dabkowska, K. (2018). Make lighting healthier. Nature 553, 274.
- Ball, H.L., and Russell, C.K. (2014). SIDS and infant sleep ecology. Evolution, Medicine, and Public Health 2014, 146.
- Starks, P., Boyden, S., and Pott, M. (2018). An Evolutionary Perspective on Night Terrors. Evolution, Medicine, and Public Health, In Press.
- Lesku, J.A., Bark, R.J., Martinez-Gonzalez, D., Rattenborg, N.C., Amlaner, C.J., and Lima, S.L. (2008). Predator-induced plasticity in sleep architecture in wild-caught Norway rats (Rattus norvegicus). Behavioural Brain Research 189(2), 298-305.
- Nunn, C.L., Samson, D.R., and Krystal, A.D. (2016). Shining Evolutionary Light on Human Sleep and Sleep Disorders. Evolution, Medicine, and Public Health 1, 227-43.
- Grandner, M.A., Hale, L., Jackson, N., Patel, N.P., Gooneratne, N.S., and Troxel, W.M. (2012). Perceived Racial Discrimination as an Independent Predictor of Sleep Disturbance and Daytime Fatigue. Behavioral Sleep Medicine 10, 235-249.