Biphasic sleep across history and modern science

Discussions of sleep patterns often contrast modern routines with biphasic sleep, an arrangement dividing sleep into two distinct periods across the 24-hour cycle. A 2025 review uses the term biphasic sleep for sleep divided into two periods across the 24-hour cycle, and monophasic sleep for a single uninterrupted night-time bout ^[8]. That review sets out a hypothesis and a framework intended to prompt new research on biphasic schedules ^[8]. By contrast, a 2021 consensus panel report defined polyphasic sleep as distributing multiple short sleep episodes across the 24-hour day, rather than one major sleep episode and possibly a minor one, a nap, each day ^[1]. The report stated that the prevalence of polyphasic sleep is unknown ^[1].

Historical accounts of first and second sleep

The historical debate focuses largely on whether preindustrial populations slept in two shifts. The historian A. Roger Ekirch, who has written on the predominance of segmented sleep in preindustrial Europe, described that pattern in a 2016 letter as common across preindustrial Europe throughout the year and not only in long winter nights, and wrote that for preindustrial Europeans waking shortly before midnight or later was thought completely natural ^[3]. He added that longer summer daylight delayed sleep onset, and the time of first rising, by at most an hour ^[3].

In the same letter Ekirch cited records of a first and second sleep from outside Europe, among them a field study published in 1953 reporting that the Tiv, subsistence farmers in central Nigeria, used the terms first sleep and second sleep as traditional intervals of time ^[3]. He attributed the consolidated night of industrialised Western societies to artificial lighting and changing cultural attitudes during the Industrial Revolution, and wrote that he was not arguing segmented sleep had been the predominant pattern among all preindustrial peoples outside the West ^[3]. However, a 2023 article in a history of medicine journal re-examined the scientific, anthropological and early modern literary sources behind the segmented sleep theory ^[7]. It noted that media reports had presented segmented sleep as potentially more natural and healthier than consolidated blocks of sleep, and asked whether the evidence might support models of early modern sleep that are not characterised by segmentation, while acknowledging that any such model is limited and uncertain ^[7].

Field observations and light experiments

Anthropological and experimental data offer differing observations regarding night waking. A 2015 study of sleep in three preindustrial societies found sleep periods averaging 6.9 to 8.5 hours, with sleep durations of 5.7 to 7.1 hours ^[4]. The sleep period was not interrupted by extended periods of waking, and naps were recorded on fewer than 7 percent of days in winter and fewer than 22 percent of days in summer ^[4].

CIBDOL · Field observations and light experiments

A different pattern appeared in another setting. A 2017 study in a rural agricultural community in Madagascar without electricity followed 21 participants by actigraphy over a total of 292 days ^[5]. The population slept 6.5 hours per night and napped on 89 percent of recorded days, and a peak in activity after midnight appeared on 49 percent of nights, which the authors described as consistent with segmented sleep ^[5]. In an experiment published in 1992, when participants were moved from a daily light period of 16 hours to one of 10 hours, their sleep episodes expanded and usually divided into two bouts of several hours, with a waking interval of 1 to 3 hours between them ^[6]. The duration of night-time melatonin secretion expanded as well ^[6].

Measured trials of polyphasic schedules

Modern polyphasic regimens attempt to distribute sleep into brief bouts across the day and night. For that report the panel screened 40,672 potentially relevant publications, reviewed 2,023 in full and retained 22 relevant papers, excluding nap and siesta schedules from its scope ^[1]. It found no evidence supporting benefits from following polyphasic sleep schedules, and its consensus opinion was that these schedules, and the sleep deficiency inherent in them, are associated with a variety of adverse physical health, mental health and performance outcomes ^[1].

Laboratory trials investigating strict polyphasic schedules report high attrition. In a prospective study, ten young healthy volunteers were restricted to one 20-minute nap opportunity at the end of every 4 hours, six sleep episodes per 24 hours, a schedule allowing a cumulative total of about 2 hours of sleep a day ^[2]. All but one stopped during the first month, and the remaining participant, a 25-year-old man, kept to the schedule for five of the eight planned weeks ^[2]. The hormone results of that study come from the one participant who stayed on the schedule, compared with his own measurements on ordinary monophasic sleep and without a statistical test ^[2]. His cortisol and melatonin release in blood appeared unaltered, while his growth hormone release fell by more than 95 percent ^[2]. The authors wrote that none of the volunteers could tolerate the intended eight weeks, even though they had exceptional motivation ^[2]. This article describes the historical record and what the cited studies measured. It does not recommend any sleep schedule.