Biphasic Sleep: What Chronobiology Says About Split Sleep

Sleeping eight hours straight is considered the gold standard, but this norm is younger than you might think. Historical sources and chronobiological data point to a sleep pattern that divides the day into two phases.

First sleep, second sleep: Historical sources

Historian A. Roger Ekirch evaluated more than 500 historical sources for his 2005 work on nighttime in pre-modern societies: diary entries, court records, medical treatises, and literature.[1] These documents regularly distinguish between a “first sleep” and a “second sleep.” Between the two lay one to two hours of wakefulness, during which people read, prayed, talked, or simply rested in semi-darkness.

Ekirch describes this pattern not as an exception but as the prevailing norm in pre-industrial European societies.[2] With the spread of artificial lighting in the 19th century, social and economic time constraints tightened, and monophasic sleep became the default. Whether this historical finding reflects a biological predisposition or was primarily a social practice remains an open question among researchers.

Ethnographic observations from three non-industrialised societies (San/Tsimane/Hadza) show a predominantly consolidated night-sleep pattern of six to seven hours without a pronounced first-sleep/second-sleep division.[3] The evidence is therefore heterogeneous: historical sources point toward biphasic sleeping, while current field studies of indigenous populations are less clear-cut.

Mono-, bi-, and polyphasic: A spectrum

Sleep patterns can be classified by the number of sleep phases per 24 hours. Monophasic refers to a single consolidated sleep period, typically at night. Biphasic means two sleep phases: usually night sleep plus an afternoon nap. Polyphasic models, where three or more short sleep phases are distributed throughout the day, are scientifically controversial because controlled long-term data are lacking.

More biologically relevant than the label is the question of whether an additional afternoon sleep phase corresponds to a biological tendency or whether it compensates for insufficient night sleep. This distinction determines whether a midday nap should be classified as a supplement or as compensation.

The afternoon dip has biological causes

Between 1 and 3 pm, reaction speed, attention, and subjective alertness drop measurably in most people. This post-lunch dip cannot be explained by digestion alone: it also occurs in people who have not eaten a meal, and it appears regardless of sleep duration and chronotype.[4]

The dip reflects a circadian-driven decrease in wake promotion. The interaction of the circadian signal and accumulated sleep pressure (Borbély’s two-process model) produces an alertness trough in the early afternoon. This trough in the alertness profile provides the mechanistic framework in which biphasic sleep makes sense.

What studies show about napping

Sara Mednick and colleagues showed in 2002 and 2003 that short afternoon naps can fully reverse an emerging performance decline on perceptual tasks.[5][6] In a study using texture-grating discrimination, performance after a nap containing slow-wave and REM sleep was equivalent to a full night of sleep after a 24-hour learning phase for this specific visual discrimination task. This finding is task-specific and not directly transferable to other cognitive domains.

Milner and Coté summarise in a 2009 review that naps of 10 to 20 minutes produce robust improvements in alertness, mood, and cognitive performance in healthy adults without substantially reducing evening sleep propensity.[7] Longer naps (45–90 minutes) containing slow-wave sleep are more strongly associated with sleep inertia upon waking but offer greater potential for memory consolidation.

A meta-analysis by Lovato and Lack (2010) confirms that naps improve cognitive functions for several hours, with the strongest effects documented in the first 30 minutes after waking.[8]

Nap length and timing: What matters

A nap’s sleep architecture depends on its length and time of day. A 10-to-20-minute nap stays predominantly in sleep stages N1 and N2 and rarely causes sleep inertia upon waking. N2 sleep is associated with the consolidation of procedural skills and the recovery of motor networks. A 30-to-60-minute nap increases the likelihood of reaching slow-wave sleep (N3), which is particularly relevant for declarative memory but also extends the sleep-inertia phase.

Timing is at least as important as length. A nap after 3 pm shifts the circadian melatonin onset and can delay the evening sleep-onset time. For people with stable sleep schedules, a nap between 12:30 and 2:30 pm is the low-risk option. If you find that late naps push your bedtime later, move the nap earlier or shorten it.

The so-called caffeine nap, where caffeine is consumed immediately before the nap, exploits the 20-to-25-minute absorption time so that the caffeine effect kicks in as sleep ends. Controlled studies show that this combination outperforms a nap alone for alertness, though sleep duration and individual caffeine tolerance vary.

Biphasic sleep as an option, not a requirement

The evidence justifies neither a blanket recommendation for nor against biphasic sleeping. What the data show: the afternoon dip is biologically real and reproducible. A short nap can correct performance decrements in this time window. It is not a sign of weakness or insufficient night sleep.

More relevant than the label “biphasic” is your individual situation. If you consistently sleep seven to nine hours at night with high sleep quality, you will benefit less from a nap than someone whose night is cut short by early rising or frequent awakenings. Biphasic sleep compensates for a structural deficit in this case, not a biological necessity.

For people whose daily rhythm shows a measurable dip in their performance curve in the early afternoon and who have the schedule flexibility, a short nap is an evidence-based tool for stabilising performance. The question is not whether the nap is scientifically legitimate, but whether it fits your own sleep architecture and daily routine.