Circadian sleep propensity

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Circadian sleep propensity (aka Process C) is a component of the sleep drive that is determined by the body clock. In a well-regulated cycle, the body clock makes sure we are most sleepy during the night. This means that circadian sleep propensity reaches its maximum during the night. See: Two-process model of sleep regulation.

The adjective circadian can also be used in the following contexts:

  • circadian cycle - ~24 hour cycle of changes in the body
  • circadian rhythm - the rhythm of physiological changes in the circadian cycle
  • circadian clock - the body clock governed by the suprachiasmatic nucleus
  • circadian phase or time or frame - time of day in reference to the circadian cycle
  • circadian shift (advance or delay) - shift in the sleep phase of the circadian cycle
  • circadian low or nadir or bathyphase or middle-of-the-night- the time in the circadian cycle when we feel most sleepy (i.e. when circadian sleep propensity reaches it maximum)
  • circadian high or peak or acrophase - the time in the circadian cycle when we feel most alert
  • circadian curve - the sine wave that represents a circadian cycle (see the picture below). The lows represent the degree of sleepiness (circadian low/nadir). The wave may be a superposition of two or more sine waves, esp. at younger ages
  • circadian preferences - preferences as to the timing of sleep (e.g. larks like to go to sleep early)(see: chronotype)

There are two nadirs in the circadian cycle in the course of one day. See: Biphasic life

This glossary entry is used to explain "Good sleep, good learning, good life" (2017) by Piotr Wozniak

Alertness changes in the course of a day
Alertness changes in the course of a day

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).