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"Results of a photoperiod experiment show that human sleep can be unconsolidated and polyphasic, like the sleep of other animals. When normal individuals were transferred from a conventional 16-h photoperiod to an experimental 10-h photo-period, their sleep episodes expanded and usually divided into two symmetrical bouts, several hours in duration, with a 1–3 h waking interval between them."
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"As for preindustrial Europeans, so too for these peoples, awakening shortly before midnight or at a later hour was thought completely natural. [...] 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. [...] More recently, ethnographic evidence from the late 19th century to the latter half of the 20th century indicates that numerous non-Western cultures not exposed to artificial lighting still experienced “first” and “second” sleep [...] What, of course, all these cultures shared with early societies in Europe [...] was an absence of artificial illumination." — Link to full paper
"People commonly increase sleep duration on the weekend to recover from sleep loss incurred during the workweek." [...] "Depner et al. show that short sleep led to later timing of energy intake, weight gain, and reduced insulin sensitivity. Weekend recovery sleep failed to prevent later timing of energy intake, weight gain, or reduced insulin sensitivity during recurrent short sleep following the weekend." — Link to full paper
"Human subjects awake preferentially at the end of REM sleep and the preparedness to awake increases with the rank number of the sleep cycle and with higher position of the circadian morning upswing of the body temperature rhythm." [...] "Awakenings after day sleep finished in the early morning is usually the most difficult and those during the day time the easiest." [...] "Being well rested is to a large extent a matter of ease awakening and reduced stage 3 and 4 sleep, and carries very little relation to subjective sleep quality."
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"The average length of the first NREM-REM sleep cycle is approximately 70 to 100 minutes; the average length of the second and later cycles is approximately 90 to 120 minutes. Across the night, the average period of the NREM-REM cycle is approximately 90 to 110 minutes" — Link to full paper
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