The search for a prehistoric sleep pattern, and any attempt to mimic it, also suffers from the same problems as the paleo diet.

An easy interpretation of sleep research is that modern life has negatively disrupted our sleep. Yet studies of preindustrial societies defy simple explanations of a sleep-paradise lost.

When Siegel and his peers reviewed the data from their 94 test subjects who live hunter-gatherer, light bulb-free lifestyles, the researchers found that they slept 5.7 to 7.1 hours per day. They did not observe segmented sleep; they found that the hunter-gatherers went to bed several hours after sunset and rarely napped; and they describe the average nightly sleep duration of 6.4 hours as “near the low end of those [of] industrial societies.”

“Sleep in these traditional human groups,” Siegel and co. conclude, “is more similar to sleep in industrial societies than has been assumed.” They speculate that segmented sleep may actually be a more recent development (from the perspective of a paleontologist) that arose when humans migrated out of Africa to higher latitudes that experience long winter nights. 

Ekirch’s exploration of sleep in Europe’s past also demonstrates that our worries about undersleeping are nothing new. In one diary entry, he notes, a shopkeeper who had expressed his determination to get 7 to 8 hours of sleep recalls returning home at 3 a.m. “not very sober” and laments, “Oh, liquor, what extravagances does it make us commit!” Britons drank alcohol and consumed opiates as sleep aids in the 17th century, and aristocrats called their (sleep-deprived) servants slothful for constantly falling asleep on the job. 

Finding a normal or ideal period to model our sleep after is made more difficult by the fact that people seem to require different amounts of sleep. Children and teenagers need more sleep than adults and seniors. Studies that subject people to sleep deprivation find that people’s ability to handle less sleep varies widely—a trait likely determined by genetics.

The search for a prehistoric sleep pattern, and any attempt to mimic it, also suffers from the same problems as the paleo diet.

Critics of the paleo diet point out that cavemen had terrible diets. Before humans cultivatedbetter food and achieved food security, vegetables were skimpy, unappetizing, and of little nutritional value, and balanced diets were rare. 

Similarly, anyone promoting paleo sleep would be thrown off by Ekirch’s descriptions of historic sleeping conditions: In 16th century Britain, peasants searched their bedding for fleas and bedbugs each night and, in order to stay warm, slept with livestock and endured their feces and urine. 

Scientists who study isolated groups—like Siegel and his peers with the Hadzas and Ju/’hoansis—are investigating how sleep may have looked in the past. They’re not looking for a model. But it’s worth noting that many of these remaining hunter-gatherers are impoverished or persecuted minorities. One group visited by anthropologists, the Toraja of Indonesia, do not exhibit segmented sleep or a single block of sleep. They wake up constantly throughout the night because “they sleep on the floor together in groups, sharing blankets and huddling close for warmth.”

Another critique of the paleo diet is that paleolithic humans’ diets varied by “season, geography, and opportunity.” 

Our best proxies of paleo sleepers exhibit similar diversity. Some enjoy segmented sleep, others sleep in one block like modern sleepers, and some groups, like the Toraja, exhibit punctuated sleep. The three groups studied by Siegel and his co-writers only slept 6.4 hours per night. But in a rebuttal in The Atlantic, sleep researcher Horacio de la Iglesia noted that other hunter-gatherer groups sleep for as many as 9 hours. When he studied two such neighboring communities, he found that one that gained access to electric lights slept an hour less each day.  

If someone suggests the paleo sleep schedule, you should ask, “Which one?”

Today I learned Sleep is a $55 billion industry. 

When we look at isolated societies or in history books for examples of more paleo-like sleep, we do so with a particular, modern perspective: that sleep is a problem to be managed or a tool that wards off drowsiness and makes us productive.

We see this in the work of Dr. Dement, who describes sleep as a key to productivity and has investigated how extra sleep allows Stanford basketball players to achieve personal bests in free throw accuracy. It’s also the foundation of the $55 billion sleep aid industry. Even contrarians who dispute the findings of researchers like Dr. Dement do so by pointing to studies that seem to show people cutting back on sleep without suffering from fatigue or a drop in productivity. 

But if there is anything to learn from paleo sleep, it’s not how many hours we should sleep or the importance of naps. It’s that there are ways to think about sleep other than as an off switch.

In many societies without electric lights, being awoken is not an obstacle to sleep. It’s a key reason why dreams are part of their lives. “In traditional non-Western societies like the Toraja,” anthropologist Tanya Marie Luhrmann writes, “what happens at night really matters. People pay close attention to their dreams, and because they are awakened more often, they have more opportunity to remember them.” Ekirch notes that Europeans once spent their inter-sleep period discussing dreams, and that the Alorese of the West Indies wake each other every night to share their reveries.