As a sleep scientist, I am often asked what "normal" sleep is. Of course, what people really want to know is whether how much they sleep is okay. That's probably why the recent study of sleep patterns in hunter-gatherer groups has garnered so much attention -- because we expect the hunter-gatherers to tell us what "natural" sleep, and therefore "normal," healthy sleep, looks like. Studies like this are interesting and important, but there are some things to keep in mind.
The study, published in Current Biology, was discussed in a recent New York Times article. This study measured sleep in three hunter-gatherer groups (two in Africa and one in South America) using an objective monitor worn on the wrist (called "actigraphy"). One interesting finding was that these people's sleep occurred entirely during the dark (few people slept past sunrise), which is unlike the behavior of many Americans (myself included). The study also found that these people spent between 6.9 and 8.5 hours in bed and slept 5.7 to 7.1 hours per night. These numbers help us to understand how much sleep varies between different cultures and environments, but there are limits to what can be extrapolated from these data and applied to Americans.
• Modern hunter-gatherers are not living fossils whose lifestyles have remained unchanged for 12,000 years.
Since our Paleo ancestors were also hunter-gatherers, then unless we invent a time machine, studying modern hunter-gatherers gives us the best idea about what life was like in the Paleolithic era (before we started farming). Still, we should not assume that the details of modern hunter-gatherers' lifestyles are identical to Paleo lifestyles. In particular, the modern groups are usually in more marginalized parts of the world because the people with guns took the more desirable land. This change in their environment could have had an impact on how, when and where they hunt and gather their food, which in turn could limit when they sleep.
• Just because hunter-gatherers are not obese does not mean that sleep is not important for health.
In the Times article, one scientist responding to the study suggests that these results call into question claims that "we need more sleep and that if you get less than 7 hours you're liable to suffer from obesity and diabetes and heart disease." This scientist was also quoted as saying, "But the average amount of sleep in these people was well under what is recommended to us as adequate sleep, and these were very healthy people who are not suffering chronic disease."
First, they are not "well under" what is recommended since 7 hours is what is recommended. Second, there is no obesity in these groups because in order to develop obesity you must consume food in excess of your caloric needs. Simply put, energy in must be greater than energy out. Modern hunter-gatherer groups rarely have access to excessive, high-calorie foods. Finally, hunter-gatherers have a shorter life expectancy. If you ignore the high rates of infant and child mortality among hunter-gatherers, adult life expectancy is probably still at least ten years shorter than in the US. So this complicates the claim that they are "very healthy."
• It is unlikely that hunter-gatherers have more control over how they spend their day (and night) than we do.
I do not consider the hunter-gatherer lifestyle a life of leisure. Although they don't have to get up in time to catch the train or spend 45 minutes in rush-hour traffic in order to get to work, they do still have to obtain and prepare food, take care of children and maintain their tools and households. All of this takes time. It may be that they don't have a choice about their bedtime or wake time due to these other competing priorities. It would have been interesting to learn whether this study collected information on what the hunter-gatherers are doing in the hours before bed and after they wake up. Of course, when people are forced to choose between sleep and their livelihoods, whether that be gathering food or going to work, they will choose their livelihoods because they need to survive.
• Americans do not necessarily sleep more than the hunter-gatherers in this study.
The article seems to be making a false comparison: sleep measured with the wrist monitor ("actigraphy") in the hunter-gatherers vs. self-reported sleep in Americans. Self-reported sleep is about one hour longer on average than sleep duration based on actigraphy. Studies in the US that used actigraphy found that average sleep duration in adults is between 6 and 7 hours, depending on the age and race/ethnicity of the group studied. There was another study of sleep among hunter-gatherer groups in Argentina that also used actigraphy; in the hunter-gatherer group that did not have electricity, average sleep duration was about 7 hours per night in the summer and 8.5 hours per night in the winter, which is longer than what is observed in the US. Finally, I measured sleep using actigraphy in a community without electricity in Haiti and found that on average they spent about 9 hours in bed and slept for 7 hours per night. Thus, some groups without electricity do sleep more than reported in the Current Biology study.
• How much sleep modern hunter-gatherers get or even how much Paleo groups got is not necessarily equivalent to how much sleep is optimal for health.
What is "normal" is not always what is optimal for health. For example, it is normal to be overweight in the US but being overweight is not optimal for health. How much a group sleeps on average may be "normal" for them in that environment but that doesn't tell us whether their sleep is optimal for their health. Furthermore, human diseases are strongly related to culture and environment. Chronic diseases, like diabetes and heart disease, became more common when we stopped hunting and foraging for food and started farming and eating more processed foods. Since Paleolithic times, our environment has changed in more ways than I can count. It takes a combination of factors to develop obesity, diabetes and heart disease. Diet and exercise are two well-known important factors, and sleep is a third one. Environmental/cultural factors could include light, noise, pollution, crowding, temperature, and social inequality. Unless you are willing to move in with one of these hunter-gatherer groups, the answer to how much sleep is optimal for your health is specific to the environment you live in.