How many hours of sleep do you need every night? If you ask a hundred random people this question, chances are a lot of them will say that about 8 hours is the optimal amount—a number that has been imprinted in the public’s mind through health campaigns, books, and articles on the topic.
Indeed, few people with some knowledge about health and disease would dispute that getting adequate sleep is important—but is eight hours really the magic number?
Until recently, the general belief about the ancestral community was that hunter-gatherers enjoyed far more sleep than we do today. Most articles on the topic tell you that our primal forebears often napped during the day and probably enjoyed at least seven to eight hours of high-quality sleep, every night, which is quite a bit more than the average of about six hours in today’s United States.5
Then, something happened. A study published in Cell found that non-Westernized people in Africa and South America—including Hadza and San hunter-gatherers from Tanzania and Namibia, respectively, and Tsimane hunter-farmers from Bolivia—only sleep on average about 6.5 hours a night (with sleep durations ranging between 5.7 to 7.1 hours).9 These surprising results suggest that hunter-gatherers don’t get any more sleep than we, people in industrialized societies, do. Moreover, they rarely take naps during the day, and researchers concurred that their biphasic sleep—a sleep pattern characterized by two distinct cycles of sleep every 24-hour period—was much less common than “Paleo wisdom” suggests.
Has the familiar idea, that hunter-gatherer ancestors delighted in more sleep than we do, been shattered? Not so fast.
Firstly, it’s hasty to assume that the sleep patterns of the traditional people undergoing the study are in fact identical with those of our ancient forebears. Plus, the approach the researchers in the study above used to measure sleep duration may have had some flaws, as other sleep experts have suggested. That said, the study makes the reasonable case that, on average, contemporary non-Western people engage in less than eight hours of nocturnal sleep.
We can’t automatically assume that the sleep habits of Paleolithic hunter-gatherers were identical to those of the study’s non-modernized people—but it’s likely, as the authors themselves note, that the observed sleep patterns are indeed characteristic of pre-modern-era Homo sapiens’ sleep patterns, and hence, may represent the types of sleep standards we are genetically suited for.
That said, unlike what many of the news articles covering this study claimed (as its findings quickly went viral), the fact that the preindustrial people under examination only get about 6.5 hours of sleep does not necessarily mean that this is the optimal amount for people in industrialized nations.
Healthy People Need Less Sleep?
Yes, healthy people may need less sleep than those suffering from chronic diseases or those with chronically elevated levels of circulating inflammatory biomarkers (systemic, chronic, low-grade inflammation). Keep in mind, I’m not just talking about severely sick individuals here.
Truth is—few, if any, people in contemporary industrialized societies are in perfect health. Not only are we exposed to environmental pollutants (pretty much everywhere we go), but most people also eat processed food, regularly, drink chlorinated water, exercise too little, and so forth. Our modern lifestyles stand in stark contrast to those of the hunter-gatherers in the aforementioned study; who live in environments that resemble the ancestral natural milieu in which the human genome evolved in, for millions of years.
There is strong evidence suggesting an association between both short and long durations of habitual sleep with adverse health outcomes.1,7,8 Furthermore, several studies have found that increases in habitual sleep times are associated with elevations in markers of systemic inflammation.2,4,6
Now, this does not verify the above hypothesis, that healthy people require fewer hours of sleep every night than unhealthy folks. Such a theory is difficult to test in clinical trials, since there are a wide range of factors that may confound the relationship between sleep duration and health status. Also, since we don’t know whether long sleep durations are a cause or consequence of poor health, a cause-effect relationship can be difficult to establish. And it could even be that there is no true relationship, but rather that other factors are inadvertently confounding the picture. For example, people who get a lot of sleep may also eat more junk food than short sleepers—clearly, this could increase levels of circulating inflammatory compounds. Researchers try to control for these types of covariates, but residual confounding often remains ineradicable.
Perhaps more relevant in the context of the above hypothesis is the connection between the immune system and sleep duration. The immune system contributes to the regulation of normal sleep, and REM sleep—a phase of sleep characterized by rapid eye movements—has been shown to be disrupted in many people with disorders involving altered cytokine concentrations.3 For example, studies in both animals and humans have shown that viral and bacterial infections may lead to increased sleep3—an increase that could be viewed as being adaptive, in the sense that it promotes recovery.
I strongly suspect, based on the literature on the topic, and my experience and observations, that healthy people in general do require less sleep than unhealthy folks. This may in part be explained by inter-individual variation in microbiota composition and the levels of circulating inflammatory mediators in the bloodstream. However, to make any strong conclusions, more research is needed.
Quality Over Quantity
Hunter-gatherers live in an untouched natural environment, get plenty of sun exposure, eat exclusively whole, unprocessed food, are never exposed to artificial lighting, and fall asleep listening to the playlists of nature. Moreover, unlike industrialized people, foragers don’t live in buildings that maintain a steady temperature of 68 degrees Fahrenheit (or 20 degrees Celsius). All these factors are important, because as the authors of the hunter-gatherer sleep study point out, the daily cycle of temperature change may be a potent natural regulator of sleep.
Our modern lives are filled with sleep disruptors—iPhones, heating systems, computers, sugar-filled junk food, hectic work-schedules, artificial lighting, pharmaceuticals, to name a few. By raising our cortisol levels, our stress hormone, and disrupting the production of the sleep hormone melatonin, these factors wreak havoc on our body’s hormonal milieu—effectively, among other things, seriously impairing sleep quality.
It’s no surprise that a lot of people in the industrialized world have problems falling asleep, waking up several times during the night and feeling sluggish and fatigued the next morning. Today’s average Joe may never experience the deep, high-quality sleep that hunter-gatherers readily access—and hence, he may require more sleep than they do.
- The sleep patterns of our Paleolithic forebears may have differed from those of contemporary hunter-gatherers.
- The fact that non-Westernized people in Africa and South America—specifically the Hadza of Tanzania, the San of Namibia, and the Tsimane of Bolivia—only get about 6.5 hours of sleep every night doesn’t mean that this is necessarily the optimal amount for everyone. Due to the fact that hunter-gatherers’ gene expression and health conditions are closer to the evolutionary norm, these people may indeed require less sleep than most people in industrialized societies. Moreover, primal people are not exposed to artificial lighting, junk food, and all of the other sleep-disrupting factors (all of which are part and parcel of modernized life), and therefore experience better-quality sleep than the average modern Joe.
- Your sleep needs may in large part be determined by your health condition and physical activity levels, and your pre-bed routine and sleep environment. If you are in great health and pay a lot of attention to optimizing the quality of your sleep (like by avoiding artificial lighting at night and stressing down before bed), you too may require less than eight hours of sleep every night.
- F. P. Cappuccio, et al. “Sleep Duration and All-Cause Mortality: A Systematic Review and Meta-Analysis of Prospective Studies.” Sleep 33 (2010): 585-92.
- J. B. Dowd, et al. “Sleep Duration, Sleep Quality, and Biomarkers of Inflammation in a Taiwanese Population.” Annals of Epidemiology 21 (2011): 799-806.
- L. Imeri, and M. R. Opp. “How (and Why) the Immune System Makes Us Sleep.” Nature Reviews Neuroscience 10 (2009): 199-210.
- M. R. Irwin, R. Olmstead, and J. E. Carroll. “Sleep Disturbance, Sleep Duration, and Inflammation: A Systematic Review and Meta-Analysis of Cohort Studies and Experimental Sleep Deprivation.” Biological Psychiatry (2015).
- D. Lieberman, The Story of the Human Body: Evolution, Health, and Disease (Vintage, 2014).
- S. R. Patel, et al. “Sleep Duration and Biomarkers of Inflammation.” Sleep 32 (2009), 200-4.
- C. Sabanayagam, and A. Shankar. “Sleep Duration and Cardiovascular Disease: Results from the National Health Interview Survey.” Sleep 33 (2010): 1037-42.
- A. Steptoe, V. Peacey, and J. Wardle. “Sleep Duration and Health in Young Adults.” Archives of Internal Medicine 166 (2006): 1689-92.
- G. Yetish, et al. “Natural Sleep and Its Seasonal Variations in Three Pre-Industrial Societies.” Current Biology 25 (2015): 2862-8.