“Biphasic sleep and human health: A theoretical paradigm for personalized sleep” [Sleep Medicine]. Seems a propos. This is a discussion of “The forgotten medieval habit of ‘two sleeps’,” which has the disadvantage of never having been spotted in lab:
Historian Roger Ekirch, drawing on ancient and medieval texts, identified a common biphasic sleep pattern in pre-industrial northern hemisphere cultures, where individuals would sleep shortly after sunset, awaken around midnight for various activities (e.g., eat, read, engage in sexual intercourse, sew, pray, or reflect and relax), and then return to sleep until dawn. This pattern, referred to as “first” and “second” sleep, reflects a culturally normative partitioning of rest.
Contradictorily, in a study of three pre-modern era hunter-gatherer/horticulturalist societies from Tanzania, Namibia, and Bolivia (all close to the equator), it was observed that they sleep continuously 6–7h per night. They initiate their sleep about 3 h after sunset and awake before sunrise, with the duration of sleep increasing by 1h from the summer to the winter season. A possible explanation for the monophasic sleep observed in this study is, as concluded by the researchers, that this pattern was “probably not present before humans migrated into Western Europe” and higher latitudes.
However, as reported by Ekirch, segmented sleep was observed, not only during winter but throughout the entire year, suggesting that it was not simply a result of longer winter nights. Furthermore, biphasic sleep has been reported in other cultures from more equatorial regions as well – from South America, Africa, and Asia and even the study by Yetish and colleagues reported that naps were observed in 22% of summer days.
Napping represents another form of biphasic sleep observed across many cultures worldwide.
I’m all for naps.
While it remains uncertain whether biphasic sleep represents the ancestral human norm, available evidence suggests that such patterns were far from uncommon across diverse historical and cultural contexts.
This variability of sleep patterns matches what we can observe in nature.
This review has examined biphasic sleep as a biologically grounded and culturally widespread sleep pattern, challenging the prevailing notion that monophasic sleep is the universal ideal. While widely promoted as the normative model, monophasic sleep may not reflect the full range of human sleep flexibility observed across historical periods, developmental stages, and diverse lifestyles. From infancy to old age, sleep naturally evolves – from polyphasic to biphasic, to monophasic, and often back to biphasic in later life – suggesting that consolidated sleep is more a product of industrialized society than a universal biological standard.
Sleep patterns come under the heading of “social reproduction,” in other words. An interesting result!