Dreams are no longer what they used to be. Or at least that is the widespread feeling. For years, there has been a warning about a silent insomnia pandemic. The problem is no longer interpreted as an individual or medical issue, but as a social phenomenon linked to long working hours, digital hyperconnection, anxiety, and crazy life rhythms. There are some data that could support this idea. In Spain, the Spanish Society of Neurology estimates that more than half of adults sleep fewer hours than recommended and that nearly 50% do not achieve restorative rest. The consumption of sleeping pills has tripled in less than 20 years, as if society had ended up turning rest — a biological necessity — into another variable to optimize, enhance, hack. Sleep is essential to eliminate the brain’s metabolic waste, improve synapses, and maximize the efficiency of cognitive processing. So if this is rapidly diminishing, we would be facing a major problem. Or not.
David Samson, evolutionary anthropologist at the University of Toronto in Canada, poses a provocative question: What if the reduction in sleep hours was not a consequence of our busy modern life, but an evolutionary advantage? He bases this idea on scientific studies and his own experience. For years, Samson studied chimpanzees, orangutans, and lemurs. He lived (and slept) with two populations of hunter-gatherers: the Hadza in Tanzania and the BaYaka in the Republic of Congo. And he found that humans are the primates that sleep the least, but live the longest, healthiest, and with the greatest cognitive capacity. This is the “human sleep paradox,” the central theme of his book The Sleepless Ape.
Samson agrees to speak with El País via video call although, he warns, he has slept little tonight. His second child is barely a week old, so his sleep is fragmented, irregular. He spends nights on crying watch. Although he admits that this time (it is his second child) they are managing much better, as they are putting into practice a custom he learned living with hunter-gatherer tribes, a somewhat unusual custom in the United States: co-sleeping. This personal anecdote brings to the table one of the book’s topics, the enormous cultural variety that exists worldwide regarding how people sleep.
Question. Sleep is essential for health and cognition, yet humans sleep less than any other primate, live longer, and are more intelligent. How is this possible?
Answer. It is a paradox. When I started researching sleep in wild great apes, in orangutans in captivity environments and later with lemurs in the lab, we were the first to accumulate data on more than 30 primate species, a number that allows very sophisticated phylogenetic analyses. That means you can take sleep patterns from the entire primate order and model them controlling for evolutionary history: brain size, body size, diet, sociology… We saw that great apes sleep around nine and a half to 10 hours. The owl monkey can reach 17 hours of sleep per day. Tarsiers sleep 15 hours, lemurs between 13 and 14. But humans… what we discovered was extraordinary: given our body size, brain size, sociology, and the fact that our life unfolds on land, the model predicts we should sleep 11 and a half hours per 24-hour period.
But humans, if you average worldwide, both in small-scale societies and large industrialized societies, sleep about seven hours. This means we have undergone a radical evolutionary experiment to become the primates that sleep the least in the entire order. And it is not because we are a wild species prone to bad habits, but because of our evolution.
Q. We don’t sleep less because of this crazy, stressful lifestyle, but because we are more efficient…
A. Exactly. If you listen to the news, you would think we are the generation of humans that has slept the worst in history, due to technology, mobile phones, and artificial light at night. But it turns out that is not the case. In an article we published last year in the scientific journal Proceedings B, we analyzed cultures worldwide and saw that small-scale cultures, like the Hadza in Tanzania or the Baka of Congo, sleep significantly less, an average of 6.4 hours, and their sleep is much more fragmented than ours. So that argument has no empirical weight. In reality, we have improved in terms of sleep.
Q. What did you learn about sleep living with these hunter-gatherer societies?
A. They sleep 45 minutes less per night on average, which is physiologically huge. Regarding sleep efficiency, in large-scale societies it is around 88%, above the 85% that the National Sleep Foundation considers high quality. In small-scale societies it is around 70, about 15 points less. They sleep less and more fragmented.
But here comes the interesting part: when you analyze the circadian function index, their circadian rhythms are stronger. Their biological clocks are much more synchronized with their environment. And that matters enormously. The Himba, who on average sleep about four and a half hours at night, have quite good health metrics in cardiovascular terms, mental health, and modern diseases. I think much of that is due to the metabolic time of all the independent clocks in their body, synchronized with their environment. They do not separate their bodies from the environment as you and I do.
Q. So, it’s not so much about how many hours you sleep but when you sleep, and if you do it following nature’s rhythm?
A. Exactly. There is very interesting research on this. An article published last year in PNAS analyzed sleep and health in 30 different countries. All showed a similar curve: sleeping very little is bad for health, but sleeping a lot, 10 or 11 hours, is also bad. However, depending on the culture, you can shift that optimal point. For some cultures, the optimum is seven or eight hours, and for others, who have perfect health, it is six. My hypothesis is that cultures that sleep less but are healthy have light diets much more in tune with their environment, and probably are not depriving themselves of the temperature and light oscillations that signal the body when to perform crucial functions.
Q. How did this happen and what were the evolutionary advantages of this change?
A. It happened about 1.8 million years ago, when human shelter innovation began. Over a few hundred thousand years, there was a gradual reduction in total sleep time: natural selection trimmed non-REM sleep. At the same time, thanks to fire, we went from chewing five or six hours a day like a chimpanzee, or eleven hours like a gorilla, to chewing about one hour a day. We not only reduced our sleep budget from eleven to seven hours, but also our chewing budget. And what we gained with that was immense additional time to socialize, strengthen bonds, hunt, learn to flintknap or use tools. We gained more time.
Q. And is this an ongoing evolution? Are we still gradually reducing the number of hours?
A. I think it would be interesting to see how far natural selection could go, but my intuition is that we have already optimized total sleep duration quite a bit. That said, there are unique genetic variants, like the DEC2 gene: if you are homozygous for DEC2, with two large copies, you can function quite well without cognitive deficit with about five hours per night. There are families with this genetic trait. Evolution may have more to do to reduce sleep, but there are things that sleep does that simply need time. Slow-wave sleep, for example, is associated with the generation of leukocytes, natural killer T cells, immune function. The system needs that time.
Q. There is a cultural and identity part in how we sleep. In Spain, the siesta is still relatively popular. Is this a good habit?
A. That is an extraordinarily complicated question. I am part of the siesta panel of the National Sleep Academy, with 30 other experts. We have been reviewing the available scientific literature and debating to establish a consensus for 18 months. I see the siesta as a supplement. If you were in Canada, in winter, and did not produce enough vitamin D because there is barely any sun, you would supplement. Similarly, if you are sleep deprived, it is probably beneficial to take a 20 to 30-minute nap. Data show that more than an hour can be harmful. The important thing is to be aware that the siesta can reduce the homeostatic sleep drive, that process of accumulating sleep pressure while you are awake. If you suffer from insomnia and wake up at three in the morning, I would recommend avoiding naps, getting up an hour earlier, and going to bed a little later to accumulate that homeostatic drive and get sleep to cross past three in the morning without waking you.
Q. In your book, you predict that in the future we will reach an era of ‘Sleep Enlightenment.’ How can we improve the way we sleep?
A. Sleep Enlightenment consists of taking advantage of what we already do well and adding what we lack. We receive a lot of criticism for our poor sleep hygiene, mobile phones, and all that. But the truth is we have physically safe and comfortable sleep environments, which allow deep and high-quality sleep. What we have lost is the connection with the outside world. If we consolidate the advances in sleep technology we have already achieved, and at the same time recover harmony with our circadian rhythms, we will be on the threshold of that Enlightenment. How? Anchor your day with morning light. Spend more than 15% of your daily time outdoors, with full-spectrum sunlight at different times of the day. If you can, walk in green environments: leaves absorb infrared light, which feeds our mitochondria. There is a chromophore in mitochondria, cytochrome C oxidase, the terminal enzyme that allows mitochondria to breathe, convert oxygen into water, and generate ATP. Without enough infrared light, we cannot feed our body. And when you are indoors, choose warm light of about 2,000 Kelvin, firelight. If we do all that and maintain the benefits of the technology we have developed around sleep, I believe we are on the edge of a Sleep Enlightenment as a species.
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