What is Normal Sleep? A look into Changing Perspectives
A Comparative Study of Sleep in Three Contemporary Pre-Industrial Societies
Recent anthropological findings are shedding new light on the question how much sleep humans intrinsically need each night. Dr. Jerome Siegel and his team from the Semel Institute of Neuroscience and Human Behavior at U.C.L.A., studied three pre-industrial traditional hunter-gatherer societies: the Hadza of Tanzania, the San of Namibia, and the Tsimane of Bolivia. These three groups represent the whole spectrum of human migration, and quite possibly evolution.
But how has our perspective on normal sleep changed over time?
Modern and pre-modern sleeping habits
Dr. Siegel and lead author Gandhi Yetish, a PhD candidate at the University of New Mexico, have reported that it seems that we, modern humans, are sleeping just as much now, as we did millennia ago. He believes the notion that we sleep less than our hunter-gatherer cousins may be more fiction than fact. His study observed three modern-day hunter-gatherer societies, and concluded that Americans, on average, slept as much – IF NOT MORE - than those individuals in these three pre-industrial civilizations. In fact, according to the study, people in these pre-industrial societies sleept a little under 6.5 hours a night, on average. They also did not nap and they did not only go to sleep when it got dark.
How pre-industrial sleep habits differ from modern sleep habits
We also see that both modern society and the three pre-industrial ones spend approximately the same amount of energy on a daily basis. Our current understanding would suggest that primitive societies might expend more energy, exercising more often, walking everywhere. This would lead to more sleep (remember, the more active you are, the more sleep you need) and better general health. Yet, this study confirms observations from previous research that shows these primitive societies were relatively fit and healthy, despite sleeping as much if not less, then we do now in North America. Studies have also shown that the daily energy spent was about the same as most Americans spend now, suggesting that physical activity is not the sole reason for their good health.
So, not only were these more “primitive” humans sleeping as much as we are now, but they were just as healthy. Established wisdom suggests that lack of sleep, independent of other factors like physical activity, is associated with obesity and chronic disease. And yet, we see these three societies were in relatively good health.
This latest research also weighs in on the established idea that natural sleep is divided into two halves of the night. From early pre-industrial writings, we knew that in the past man would sleep from sundown until midnight, and then from 2 or 3am until sunrise, waking up in the middle of the night for 2-3 hours. Napping for an hour or so in the middle of the day was considered normal, even advisable. The accepted thought was that human sleep is not naturally consolidated into 7-8 hour episodes, that this “need” to sleep one solid block is a byproduct of “modern” living. Our lifestyles (work and recreation) demand full participation in society from sunup until bed time, and with the invention of the electric light bulb, our bed times became progressively later and later, as work and recreation kept us awake longer and longer. Therefore, sleep had to become consolidated into one solid block to accommodate later and later bed times.
Debunking sleep myths
However, this study also debunked that concept. The observations from the current research are that these hunter-gatherers also slept in one consolidated sleep period, mirroring our very own sleep patterns. So here we have three societies, without electric lights, without modern technological distractions, staying instinctively awake well into the night and, roughly, going to sleep and waking up at the same time (or even slightly earlier) than we do now.
The reason postulated for this is as profound as it is simple. Temperature stimuli are the driving factor in sleep initiation and consolidation, superseding light cues. We have always believed the notion that sleep and sleepiness were triggered by light cues, specifically the approaching darkness. As the sun set and darkness came, ancient man would yawn, signaling the sleep period. As the sun rose in the early morning, the light would wake us up and the day would start anew.
However, the current observations are more consistent with the concept that temperature is the main factor in initiating sleep. As body temperatures begin to fall, a few hours after the sun has set, sleepiness sets in. These societies went to sleep almost 3.5 hours after sunset. As the temperatures CONTINUED to drop, sleep continued unabated. Then as the ambient temperature reached its nadir, these people would wake up, just as the sun started to rise. Importantly, they were awake to get early morning sunshine. This idea has important implications related to the causes and treatment of insomnia.
I think it is reassuring to see that whether primitive or modern we, as humans are more alike than we are different. Even in sleep, we see similarities in our behaviors. We sleep about the same amount of time, we go to bed and wake up in the same manner, and I will bet we dream the same or at least similar dreams. Dreams during which we make a better life for our loved ones and survive the next day, either dodging urgent emails in the office or running from tigers in the savannah or snakes in the forests.
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