This is the first official .2 Newsletter! This week we take a look at sleep, why sleeping on the ground was dangerous for humans in the past, the implications of those dangers, and how to incorporate ancestral sleep insights into our own contemporary lives.

Moving forward, expect to receive the full article version of my YouTube videos every Monday, in addition to some other goodies I’ll be sharing here and there (currently working on an exercise mini-course free for all you lucky subscribers).

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The Adaptive Scholar

How To Get Better Sleep (Insights From Human Evolution)

Intro

We can begin our discussion on sleep, by comparing it to something like fasting. Fasting from food intermittently or for prolonged periods of time is a common health trend in the modern world. Health professionals have begun recommending this rather unorthodox eating pattern due to its health benefits which include visceral fat loss, improved blood pressure, and even enhanced memory [1].

What virtually no doctor is prescribing, though, is that you fast from sleep. Despite the mysterious nature of sleep, we know that it is essential to one’s overall health and sanity. It doesn’t take much time before our bodies begin to fail and our minds begin to stray simply from a lack of it. In fact, studies show that insufficient sleep duration and quality are linked to [2]:

1. Increased mortality risk.

2. Weight gain and obesity. 

3. Increased risk of diabetes.

4. Increased inflammation.

5. Cardiovascular disease.

6. Reduced neuro-cognitive functioning.

7. Mood and anxiety disorders.

Considering that up to 70 million people in the United States alone report having a sleep disorder [3], inferably leaving them susceptible to any of those conditions, there is much cause for concern. Solutions to this problem should be explored down every avenue of scientific research. 

Unfortunately, many of the proposed solutions are pharmacologically biased. Indeed, medicine can often be effective in treating sleep disorder symptoms, but it rarely prevents the underlying behavioral issues. For this reason, exploring sleep from the perspective of behavioral evolution should be considered. This will help us understand why so many people have this problem, and how to solve it with behavioral adjustments rather than exogenous chemicals.

Sleeping Through the Ages

First, it’s important to understand how human sleep compares to our primate relatives from which we share a common ancestor. Then, we can look at how human sleep behavior is unique. This will give us a rough timeline of how our current sleeping patterns evolved to be.

The earliest primates, resembling modern day galagos, were probably small, nocturnal, and lived in the trees. Phylogenetic analyses suggest that they likely slept in tiny tree-hole nests [4]. As some primate species evolved, many of them grew bigger, and they could no longer fit in those small tree holes. They resorted to sleeping on branches like we see in many modern-day monkeys. Still, some grew even bigger.

Because of this increase in size, sleeping on a single branch became dangerous, increasing the risk of a catastrophic fall. This triggered an important evolutionary transition in the ancestor of all great apes. Humans, chimpanzees, gorillas, and orangutans all build nests or platforms to sleep on, which is believed to be a 14-18 million-year-old behavior [5]. By creating this platform the great apes of the trees can increase stability and reduce the risk of falling.

We, on the other hand, build our platforms on the ground, which marks the transition to uniquely human sleeping patterns. It is argued that fire is what enabled us to make this terrestrial transition by providing protection from predators [6]. There is one aspect of human sleep that is of utmost importance, though - of all the primates, our sleep seems to be the most efficient. Let’s look at these two factors (fire and efficiency) in greater depth.

The Architecture of Human Sleep

Like much of our anatomy and psychology, our sleep architecture has been sculpted by evolution. Accordingly, natural selection has favored sleep behaviors which led to greater reproductive success for past humans. The transition to terrestrial life, as noted above, changed the selective pressures placed on humans, which then had downstream effects on our sleep. 

First, sleeping on the ground made us more vulnerable to many large-bodied predators that otherwise could not reach us in the trees. Fire likely helped make these predators more visible in the night and probably scared many of them away. It’s smoke would have also deterred pathogen-spreading insects like mosquitoes 

Second, the fire itself would have come with some costs as well. Most obviously, it would have attracted other human groups, making intergroup raids more of a threat. 

Third, by extending light into the night, fire would have provided more time for groups to socialize. It’s been shown that hunter-gatherers today spend 84% of their nighttime conversions around fire engaged in storytelling [7]. This fire talk would have increased social bonds. As a result, these tighter bonds could have been evolutionarily advantageous because group cohesion would have increased survival, especially under conditions of intergroup conflict. 

Time spent sleeping may have been replaced with time spent socializing, which brings us to the point of sleep efficiency. 

Taking all these factors into account, researchers David Samson and Charles Nunn [8] proposed the “Sleep Intensity Hypothesis”. They show how humans, when compared to all other primates, spend the least amount of time asleep (7 hours on average), but spend the greatest percentage of that time in rapid eye movement (REM) sleep - a portion of the sleep cycle considered essential for creating meaning associations and learning spatial information.

Accordingly, they claim that the combination of increased predation, increased intergroup conflict, and the incentivization of more social time put the necessary selective pressures on ancient humans to consolidate their sleep into shorter, more efficient time-spans: 

Insomnia and the Sentinel Hypothesis

Now, how can this evolutionary framework help explain why people have sleep problems today? Insomnia is a sleep disorder defined by the inability to fall and/or stay asleep. It is often associated with stress and a hyperactive nervous system. It is a growing trend in the industrial world, but researchers Patrick McNamara and Sanford Auerbach suggest that it stems from our past. 

They propose that insomnia-like tendencies are an adaptation to increased night-related threats [9]. Again, by increasing the threat of predation, the transition to living on the ground would have incentivized ancient people to “sleep with one eye open”. Studies on modern-day Hadza hunter-gatherers support this notion, showing that only .2% of the time, all members of the tribe are sleeping simultaneously [10]. In other words, for 99.8% of the “night hours”, at least one group member was awake. 

Now known as the Sentinel Hypothesis, asynchronous sleep patterns between individuals in the group evolved because it helped prevent external threats. The problem lies in that, while we still sleep terrestrially, we do it in safe conditions that are mostly unexposed to predators and violent humans. Our biology may have been programmed to be on edge while we sleep, but this is no longer a serious concern for many of us. As a result, sleeping itself may be difficult for some.

Tips from a Hunter-Gatherer

Although these sentinel sleep interruptions are present in modern hunter-gatherer communities, they don’t seem to develop pathological insomnia like people in industrialized countries. According to a study on three separate hunter-gatherer communities, only about 2% of people said they experienced sleep issues regularly (compared to 10%-30% in industrialized societies) [11].  In fact, there were no words for things like insomnia in any of their languages. 

What do they do differently? The authors suggest two factors that play a role in their arguably more sound sleep. First, they’re sleep patterns are largely dictated by the natural temperature fluctuations of their environments, with sleep onset associated with the cooling periods of the evening. Being that we often live in temperature-controlled environments, this lack of temperature awareness may contribute to our different sleeping behaviors.

Second, the maximal amount of sunlight they expose themselves to occurs early in the morning. By noon they seek shade. The researchers of this study suggest that:

By activating the part of the brain that controls one’s circadian rhythm (i.e. the suprachiasmatic nucleus), this maximal morning sunlight triggers the hormonal biology that regulates our sleep habits. 

Lastly, sleep scientist Matthew Walker has developed science-backed tools to help people achieve better sleep. What I find interesting about many of his tools, is that they are rejections of modern trends. They are tools to help mimic ancestral sleep environments. For example, he is a proponent of avoiding digital light before bed and he even suggests keeping clocks out of the bedroom [12]. Neither of which were present in human sleep environments until very recently

Is it such a surprise that altering the world around us so much and so quickly could be disrupting our sleep? I don’t think so.

Conclusion

Clearly, neglecting sleep is a problem we want to avoid. There are far too many health risks. By looking at the evolution of human sleep we see specific factors, such as sleeping on the ground and the use of fire, that cause us to develop a unique sleep architecture when compared to other primates. This led to downstream effects, one of which is increased sleep efficiency, but the other is a propensity for insomnia-like symptoms. 

While it is a fallacy to assume that “just because hunter-gatherers do it, we should do it too”, these cultures may offer us some insights as to how we can improve our sleep. Simply, consider dropping the temperature in your house before you want to fall asleep and view your morning sunlight. Also, avoid screens and clock-watching before bed.

Sweet dreams. 

References:

[1]  https://www.hopkinsmedicine.org/health/wellness-and-prevention/intermittent-fasting-what-is-it-and-how-does-it-work 

[2] Grandner, Michael. 2017. “Sleep, Health, and Society.” Sleep Medicine Clinics 12(1):1-22.

[3] National Institutes of Health. (2003) Revision of the NIH National Sleep Disorders. 2003 Research Plan. Bethesda, MD: National Institutes of Health.

[4] Kappeler, PM. 1998. “Nests, tree holes, and the evolution of primate life histories.” American Journal of Primatology 46(1):7-33.

[5] Duda, Paval and Jan Zrzavý. 2013 “Evolution of life history and behavior in Hominidae: towards phylogenetic reconstruction of the chimpanzee-human last common ancestor.” Journal of Human Evolution 65(4):424-46.

[6] Wrangham, Richard W. 2009. Catching Fire: How Cooking Made Us Human. New York, NY: Basic Books

[7] Wiessner, Polly. 2014. “Embers of society: Firelight talk among the Ju/’hoansi Bushmen.” Proceedings of the National Academy of Sciences 111(39):14027-14035.

[8] Samson, David and Charles Nunn. 2015. “Sleep intensity and the evolution of human cognition.” Evolutionary Anthropology 24(6):225-37.

[9] McNamara, P., and Auerbach, S. (2009). Evolutionary medicine of sleep disorders: Toward a science of sleep duration. In Evolution of Sleep: Phylogenetic and Functional Perspectives (pp. 107-122). Cambridge University Press.

[10] Samson, David et al. 2017. “Chronotype variation drives night-time sentinel-like behaviour in hunter-gatherers.” Proceedings of the Royal Society: Biological Sciences 284(1858):20170967.

[11] Yetish, Gandhi et al. 2015. “Natural sleep and its seasonal variations in three pre-industrial societies.” Current Biology 25(21):2862-2868.

[12] The Matthew Walker Podcast - #25: Optimising Your Sleep

Fit Fuel Song Suggestion

The Fit Fuel song suggestions are hand-picked by yours truly to elicit the motivation (and possibly aggression) needed to initiate or persist through a grueling workout. They consist of heavy, brutal guitar riffs and gruesomely guttural vocals. Additionally, I timestamp what I believe to be the best riff of the song - one that will kick your nervous system into overdrive when approaching a personal record.