Segmented Sleep in Preindustrial Societies.

I was intrigued to learn of the study conducted of three preindustrial cultures, without access to electric lighting, by a team of researchers led by Dr. Jerome Siegel in an effort to determine how humans slept “before the modern era.” Titled “Natural Sleep and Its Seasonal Variations in Three Pre-industrial Societies,” it appears in the November issue of Current Biology.1 Having written on the predominance of “segmented sleep” in preindustrial Europe,2 I was particularly surprised by the discovery reported by Yetish et al.1 that the members of all three of these equatorial societies did not “regularly awaken for extended periods in the middle of the night.” In short, these individuals did not experience a “bimodal sleep pattern.” The authors conclude, “by extension,” that this pattern was “probably not present before humans migrated into Western Europe. Rather, this pattern may have been a consequence of longer winter nights in higher latitudes.” Not only is this broad inference highly questionable, but significant historical and ethnographic evidence also exists to suggest the prevalence of segmented sleep in preindustrial equatorial cultures. First, segmented sleep was common across preindustrial Europe throughout the year, not just during long winter nights. Whether in northern England or southern Europe, this pattern of sleep reflected, at most, the limited effect of seasonality, at least in countries located south of northern Scandinavia where seasonal variations in the availability of natural light were pronounced. Even in the “siesta cultures” of Spain and Italy, seasonal variations were modest despite reliance on napping to combat the intense midday heat. During the summer months, some men and women, to be sure, were apt to work or socialize later at night, but longer hours of daylight ordinarily extended sleep onset and, in turn, the time of “first rising” by at most 1 h. As in many preindustrial cultures, sleep onset depended less on a fixed timetable than on the existence of things to do. In the winter, whether for conviviality or work, preindustrial households remained active well after sunset, not retiring until 21:00 or 22:00, or later when visiting with neighbors. People relied on primitive illuminants, such as rushlights and oil lamps, or on the natural light of the moon and stars on clear nights, even to perform unskilled chores such as chopping firewood.3 Second, it is clear that biphasic sleep was not unique to Western households. Instead, it occurred well beyond the bounds of Europe and North America in other cultures and continents, including the Middle East, Africa, South Asia, Southeast Asia, Australia, and Latin America, thereby heightening the likelihood that throughout the preindustrial world this form of sleep was not at all uncommon, including in equatorial cultures.4 The French priest André Thevet, on traveling to Rio de Janeiro, Brazil in 1555, reported that the Tupinamba Indians ate whenever they had an appetite, “even at night after their first sleep they get up to eat and then return to sleep.” 5 In the early 19th century, residents of Muscat, the capital of Oman, were said to retire early, lying “down before pii: sp-00644-15 ht tp://dx.doi.org/10.5665/sleep.5558