Biphasic sleep
Biphasic sleep is a sleep pattern composed of two sleep episodes within 24 hours: a night sleep and a siesta.
Biphasic sleep is one of the two recommended sleep practices (the other is monophasic sleep). In biphasic sleep, nighttime sleep period is complemented with a well-timed siesta (as practiced in Mediterranean countries). In a healthy biphasic pattern, nighttime sleep period will last 3-8 hours, while the siesta nap will last 10-180 minutes. Siesta should follow nighttime sleep within 7-9 hours from natural waking. Early siesta isn't fully restorative. Late siesta may result in a phase shift. Biphasic sleep is the most natural form of sleep that develops in free running sleep.
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This glossary entry is used to explain "Good sleep, good learning, good life" (2017) by Piotr Wozniak
Figure: Changes in alertness in the course of a day (i.e. in the course of a 24-hour circadian cycle). Alertness is expressed here as learning performance. Sleep data come from a SleepChart log. Learning data come from SuperMemo. Best learning performance occurs early in the morning. There is a second peak of good performance in the evening (13-17 hours from waking). Sleepiness is the opposite of alertness. Optimum time for a siesta nap occurs in the 8th hour since waking. The two dips in alertness correspond with optimum times for sleep in a biphasic sleep cycle. Horizontal axis corresponds with the circadian phase, i.e. the number of hours since awakening in the subjective morning. 0 on the horizontal axis corresponds with circadian Phase 0, i.e. the optimum/natural waking time. Vertical axis corresponds with the average recall in learning based on spaced repetition. Blue dots express recall at a given circadian phase/time (in percent). Thick blue line is the approximation of the circadian alertness derived from a two-process model of sleep regulation (inspired by Alexander Borbely). Learning performance (thinner line) provides a good match to circadian alertness (thicker line).