Currently, health experts recommend that adults aim for about eight hours of continuous sleep in dark, cool environments. Achieving this goal consistently, they say, can add years to a person’s life expectancy.
However, this ideal resting condition often remains unattainable for many individuals. Furthermore, according to the book “The Monkey with Insomnia: The History of Sleep in Human Evolution”, released in May, this was not the predominant form of sleep during much of humanity’s evolutionary trajectory.
To unravel the mysteries of the evolution of human sleep, anthropologist David Samson, author of the work and associate professor at the University of Toronto, delved into complex research. He climbed trees to investigate chimpanzee roosting sites and visited remote communities.
Samson’s findings show a significant transformation in human sleep patterns, which have become shorter but more intense and adaptive than those of our primate ancestors. This crucial shift freed up valuable time for activities such as toolmaking, complex social interactions, and the expansion of the species across different regions of the planet.
The expert argues that these unique sleep patterns were fundamental to survival, drove innovation and shaped essential behaviors of our species. Samson adds that contemporary people, often suffering from sleep deprivation, have a lot to learn from the rest habits of their ancestors. “Sleep hugely impacts our mental and physical performance throughout the day,” he explains to CNN. “How is it possible, then, that we are the primates that sleep the least amount of time on the planet?”
This interview has been edited for clarity and brevity.
Understand the origin of the title “The Monkey with Insomnia” in the sleep study
The work “The History of Sleep in Human Evolution” was published in May by Princeton University Press.
David Samson explains that it took approximately 15 years to accumulate sufficient data on primate sleep and perform the necessary statistical analyses. These analyzes indicated that humans, evolutionarily, should sleep about ten and a half hours a day. He notes that this estimate contrasts sharply with the actual average human sleep, which in many cultures hovers around seven hours, establishing our species as a notable evolutionary exception.
Not only do humans have the shortest sleep duration among primates, they also have the highest proportion of REM (Rapid Eye Movement) sleep on the planet. The book sets out to investigate and elucidate the trajectory that led to this particularity.
Humans’ transition to sleeping on the ground and its evolutionary advantages
When describing this change, Samson compares the new sleeping environment to a shell, an innovative and completely different space.
Evidence suggests that early humans, like Homo erectus, practiced group sleeping. It is likely that, during this period, there was already a controlled domain of fire. Due to the presence of larger groups, with demographic diversity including the elderly and teenagers, it was common for there to be someone on guard 24 hours a day, ready to warn of dangers. Samson highlights that a hunter-gatherer camp was adapted to have individuals with varied sleep patterns (morning and night) well distributed, making the camp’s “cocoon” safer throughout the day. This allowed individual members of the group to enjoy deeper, more restful sleep, relying on collective vigilance, a value that modern societies often lose.
Many readers may associate ideal sleep with the need for isolation from people and stimuli. However, in almost all of the small-scale environments that Samson has researched, intense dynamics predominate. The general feeling of security upon returning to camp was like a bubble, a protective cocoon, where one could finally let one’s guard down.
Why REM sleep is more abundant in humans despite shorter total duration
Samson points out that during phasic REM sleep, the person deeply disconnects from the outside world, reaching a peak of vulnerability. For human ancestors, being in this state represented a high cost. However, with the construction of the aforementioned protective “shell” of camps, the opportunity arose to obtain a greater proportion of this valuable sleep than other animals.
REM sleep, known for its association with dreams, is also linked to creativity and innovation, qualities that, according to the anthropologist, were essential prerequisites for the success of our species.
The importance of studying chimpanzee nests in understanding human sleep
Samson reports climbing countless trees for his studies. Great apes are unique in that they build nests that serve as beds in trees, a fascinating habit. These nests offer warmth, protection against large predators because they are not on the ground, and defense against micro-predators because they are made from insect-repellent plants.
A chimpanzee rests in a nest to sleep in the top of a tree in Gombe Stream National Park, Tanzania.
Measuring these “beds” directly in their natural environment (“in situ”) has provided valuable insights into the workings of the chimpanzee mind. These observations were crucial to understanding the challenges our human ancestors faced when they began sleeping on the floor, losing this peculiar resting place.
What Tanzanian hunter-gatherers reveal about sleep and the circadian rhythm
Samson suggests that, in the West, there is a kind of “fetishization” of sleep. When questioning hunter-gatherers about their taste for sleep, the unanimous answer is: “I love my sleep.” He considers this a mystery, as he empirically knows that their sleep is more fragmented. The key to this apparent contradiction, he believes, lies in our biological clock: circadian rhythms. Most people spend more than 90% of their time indoors over a 24-hour period, which is harmful.
Samson argues that unless we understand how to enhance and enjoy the natural rhythm of our biological clocks, our relationship with sleep will always be strange, potentially dysfunctional, or dysregulated.
How ancestral sleeping habits impacted the anthropologist’s personal life
The anthropologist reports that his own relationship with light significantly improved his sleep. He ditched alarm clocks and allows the sun to wake him up naturally, without opaque curtains that disconnect him from the outside environment. When he wakes up, he goes out and has breakfast in the sunlight. At noon, even on cloudy, rainy or cold days, he makes a point of going out, an advice he shares.
At night, sunset serves as your main cue to reduce exposure to blue light from artificial sources like screens. He uses his cell phone in dark mode, for example. When moving around the house, a habit he has maintained for years, Samson uses small flashlights with a color temperature of 2,700 kelvins or less, which emit warm light and can be charged via USB. Candlelight is even more effective. In addition, he monitors his metabolic rate, eating his last caloric meal three to four hours before his scheduled bedtime.
For those who have difficulty sleeping, Samson advises less concern about sleep itself and more about the question: “Am I in evolutionary synchrony or evolutionary dyssynchrony?” Insomnia is a classic example of evolutionary incompatibility, where the basis of our evolution was how things were, not how they are now. Insomnia is a manifestation of hypervigilance. Our ancestors also had it, and there was a good reason for that, as they lived in dynamic and dangerous environments. We are their descendants, and this predisposition is still with us, a valuable element that is often ignored in modern sleep medicine.
Samson concludes that we are on the verge of a revelation about sleep. We gain countless benefits in terms of comfort and safety in our resting place, but, on the other hand, we lose connection with our circadian physiology. He believes that by bringing these two together, we can improve our sleep and well-being in the future, grounded in both an understanding of our evolutionary history and modern sleep science.