How much sleep does an adult human need? If you ask a hundred people on the street this question, chances are a lot of them will say that about 8 hours every night is the optimal amount: a number that has been imprinted in the public’s mind through health campaigns and books and articles on the topic. Few people with some knowledge about health and disease are going to disagree that getting adequate sleep is important; but is 8 hours really the magic number?
Has the idea that our hunter-gatherer ancestors got more sleep than we do been shattered?
Up until very recently, the general belief within the ancestral health community was that Paleolithic hunter-gatherers got far more sleep than we do today. Most articles on the topic would tell you that our primal forebears often napped during the day and probably enjoyed at least 7-9 hours of high-quality sleep every night, which is quite a bit more than the average of about 6 hours in the U.S. today (1).
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 Nambia respectively, and Tsimane hunter-farmers from Bolivia – only sleep on average about 6.5 hours a night (sleep durations of 5.7–7.1 hr) (2). In other words, they don’t get any more sleep than people in industrialized societies. Some sleep “experts” have argued that the approach the researchers in this study used to measure sleep duration may have some flaws (3). That said, I think, based on the results, that it’s safe to conclude that these non-westernized people do indeed get less than 8 hours of sleep every night.
The study also looked into other aspects of these people’s sleep habits. Among other things, it found that hunter-gatherers rarely take naps during the day, and that biphasic sleep – a sleep pattern characterized by two distinct cycles of sleep every 24-hour period – is much less common among these peoples than “Paleo wisdom” suggests.
While we can’t automatically assume that the sleep habits of Paleolithic hunter-gatherers were identical to that of the pre-industrial people included this study, it’s likely, as the authors of the study say, that the sleep patterns observed in this study are indeed characteristic of pre-modern era Homo sapiens, and hence, may represent the types of sleep patterns we are genetically suited for. That said, unlike what many of the news articles covering this study claims (the study quickly went viral after it was published), the fact that the traditional people in this study only get about 6.5 hours of sleep doesn’t necessarily mean that this is the optimal amount for people living in industrialized nations.
There are two primary reasons why I make this statement…
Healthy people probably need less sleep
It has become increasingly clear to me that “health status” is an important determinant of sleep requirement, in the sense that healthy people probably need less sleep than individuals who have chronically elevated levels of circulating inflammatory biomarkers (systemic, chronic low-grade inflammation), suffer from chronic disease, or otherwise are in bad shape. Keep in mind, I’m not just talking about severely sick individuals here…
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 on a regular basis, drink chlorinated water, exercise too little, and so forth. This is in stark contrast to the hunter-gatherers included 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 to suggest that there is an association between both short and long duration of habitual sleep with adverse health outcomes (4, 5, 6). Furthermore, several studies have found that increases in habitual sleep durations are associated with elevations in markers of systemic inflammation (7, 8, 9).
These studies suggest that there is an association between health status and sleep duration. That said, they don’t prove that the hypothesis that healthy people require fewer hours of sleep every night than unhealthy folks is necessarily true. This hypothesis 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. People who get a lot of sleep may for example eat more junk food than short sleepers, something that could clearly increase the levels of circulating inflammatory compounds in their blood. Researchers try to control for these types of covariates, but residual confounding often remains. Another problem is that it can be tough to establish a cause-effect relationship, since we don’t know whether long sleep durations are a cause or consequence of poor health.
Perhaps more relevant in the context of the hypothesis above is the connection between the immune system and sleep duration. The immune system contributes to the regulation of normal sleep, and REM sleep have been shown to be altered in many disorders that involve altered cytokine concentrations (10). For example, studies in both animals and humans have shown that viral and bacterial infections tend to increase sleep needs (10). The increases in sleep duration observed during infection is likely adaptive in the sense that it promotes recovery.
I strongly suspect, based on the literature on the topic, as well as my experience and observations, that healthy people in general require less sleep than unhealthy folks. This may in part be explained by inter-individual variation in the levels of circulating inflammatory mediators. The sleep needs of each individual can also change over time, due to shifts in health status. You have probably experienced this effect for yourself. In periods when you are sick, depressed, or otherwise feel weak, you probably feel the need to sleep more than when your body is in good shape.
Besides health status, another important determinant of sleep requirements is physical activity levels. Someone who does a lot of high-intensity exercise may require more sleep than a sedentary person, as his body needs peace and quit in order to recover. Hunter-gatherers are very physically active, but they engage mostly in low-intensity aerobic activities; hence, they don’t need a lot of extra sleep in order to recover from exercise.
It’s not just the duration of sleep that matters, but also the quality
Hunter-gatherers live in an untouched natural environment, spend a lot of time in the sun, eat exclusively whole, unprocessed food, are never exposed to artificial lighting, and fall asleep listening to the sounds of nature. Moreover, unlike industrialized people, foragers don’t live in buildings that maintain a steady temperature of 68 degrees Fahrenheit (ºF) (20 °C). This may be important, because the daily cycle of temperature change is likely a potent natural regulator of sleep.
Our modern lives are filled with things that can disrupt our sleep, including iPhones, heating systems, computers, sugar-filled junk food, hectic work-schedules, artificial lighting, and pharmaceuticals. These things wreak havoc on the hormonal milieu in our body by raising the levels of the stress hormone cortisol and disrupting the production of the sleep hormone melatonin, thereby seriously impairing sleep quality.
In other words, it’s no surprise that a lot of people in the industrialized world have problems falling asleep, wake up several times during the night, and feel sluggish and fatigued when they wake up. The average Joe may never experience the deep, high-quality sleep that hunter-gatherers get; hence, he may require more sleep than they do.
- The sleep patterns of Paleolithic hunter-gatherers may have differed from to that 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. Hunter-gatherers may require less sleep than most people in industrialized societies, due to the fact that they are in better health. Moreover, they are not exposed to artificial lighting, junk food, and all of the other sleep-disrupting factors that are a part of life in the industrialized world, and therefore experience better quality sleep than most industrialized people.
- Your sleep needs may in large part be determined by your health condition and physical activity levels, as well as 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 (e.g., by avoiding artificial lighting at night and stressing down before bed), you may require less than 8 hours of sleep every night.