We’ve heard it all our lives, a good night’s sleep is at least 8 hours, 8 continuous hours. Although it’s not uncommon for people to wake up in the middle of the night for a while, when that happens, it’s very likely to be categorised as insomnia. But what if that were a perfectly fine way of resting? What if taking a break in your night’s sleep were, in fact, the norm?
Several historians, psychiatrists and neuroscientists are arguing that, in pre-industrial times, people regularly woke up for 1-2 hours in the middle of the night. Instead of staying in bed, waiting to fall back asleep, they got up, did some sort of activity, then went back to sleep later for another few hours.
Roger Ekirch, a professor of history at Virginia Tech who researches segmented sleep and wrote “At Day’s Close: Night in Times Past” on the subject, found evidence as early as Ancient Greece of “first” and “second sleeps”. Homer’s “Odyssey” records such instances as far back as late 8th to early 7th century BC. “The references were stated as if segmented sleep was utterly natural and did not need to be explained”, Ekirch said.
He argues that, in pre-industrial times, people went to bed earlier, at sundown, slept for several hours and woke up sometime after midnight for meditation, sex and socialisation before returning to sleep for a few more hours. According to him, the shift to one continuous sleep per night was fostered by the emergence of electricity and, implicitly, artificial light allowing people to work into the later hours of the evening, and alarm clocks, waking people up regardless of sunrise.
Thomas Wehr, a psychiatrist and scientist emeritus of the US National Institute of Mental Health, also found in a 1992 study that, isolated form mundane activities, people would naturally shift to segmented sleep cycles. Participants to the study spent 14 hours a day in a dark room. After a while, most of them went to sleep early in the evening, woke up at some point during the night, then fell back asleep before early morning. “On average, for the whole group, it was bimodal”, Wehr explained. “The average pattern was very similar to sleep in some diurnal, day-active animals like panthers.”
According to a chapter on the “Evolution, Chronobiology, and Functions of Polyphasic and Ultrashort Sleep” in “Why we nap”, humans and primates are among the minority mammal species that mainly sleep in one go. The authors found that over 86% of mammals, including dogs, rodents, hedgehogs and even certain whales, have polyphasic (in several stages) sleeps.
Wehr’s study could also indicate an evolutionary element of segmented sleep. He found that, on average, there was at least one person of the group awake at any given time, suggesting a “sentinel” survivor function, where one individual of a group has to keep watch at all times.
On the other hand, Jerome Siegel, a professor of psychiatry and biobehavioural sciences at UCLA, argues against the evolutionary aspect of polyphasic sleeping. To prove his point, he researched sleeping patterns of modern hunter-gatherer societies in Tanzania, Bolivia and Namibia. These isolated populations’ way of life is similar to that of prehistoric humans and the study found that, individual of each other, modern hunter gatherers sleep an average of 5.7 to 7.1 continuous hours per night.
“The bimodal sleep pattern that may have existed in Western Europe is not present in traditional equatorial groups today and, by extension, was probably not present before humans migrated into Western Europe”, Siegel argues. “Rather, this pattern may have been a consequence of longer winter nights in higher latitudes.”
Moreover, some researchers point out that determining the frequency of polyphasic sleep in modern societies is challenging, as there is no strict definition. Whether or not naps or brief nighttime waking moments should be taken into account is still up for debate.
In the meantime, Daniel Buysse, a professor of psychiatry, medicine and clinical and translational science at the University of Pittsburgh, suggests that there is no one true human sleep pattern and that environmental and social factors, such as geography or seasonality, play a great role in the way humans around the world sleep.
“I mainly don’t think that there’s any one pattern of sleep that is the human sleep pattern”, Buysse argues. “I think that adaptability is the main feature.”