Circadian cycle is the cycle of physiological changes in the body in the course of a single day. It is better known under a less accurate name circadian rhythm. One of the most important roles of the circadian cycle is to determine the timing of sleep, which should ideally coincide with natural darkness of nighttime. An undisrupted circadian cycle is usually a bit longer than 24 hours. Hence the name circa-. The cycle is reset to match 24 hours on the clock primarily in the morning with the help of sunlight and other zeitgebers. Sleep occurs when maximum circadian sleep propensity combines with a high homeostatic sleep propensity.
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).
Figure: Optimizing the timing of brainwork with respect to the circadian cycle. This exemplary graph was generated with the help of SleepChart on the basis of 3-year-long daily measurements of a free-running sleep rhythm. The horizontal axis expresses the number of hours from awakening (note that the free running rhythm period is often longer than 24 hours). are actual sleep episode measurements with timing on the horizontal, and the length on the left vertical axis. Homeostatic sleepiness can roughly be expressed as the ability to initiate sleep. Percent of the initiated sleep episodes is painted as a thick blue line (right-side calibrations of the vertical axis). Adenosine-related homeostatic sleep propensity increases in proportion to mental effort and can be partially cleared by caffeine, stress, etc. Circadian sleepiness can roughly be expressed as the ability to maintain sleep. Average length of initiated sleep episodes is painted as a thick red line (left-side calibrations of the vertical axis). Mid-day slump in alertness is also circadian, but is biologically different and results in short sleep that does not register as red sleep maintenance peak. Sleep maintenance circadian component correlates (1) negatively with temperature, ACTH, cortisol, catecholamines, and (2) positively with: melatonin and REM sleep propensity. Optimum timing of brainwork requires both low homeostatic and circadian sleepiness. There are two quality alertness blocks during the day: first after the awakening and second after the siesta. Both are marked yellow in the graph. For best learning and best creative results use these yellow blocks. Caffeine can only be used to enhance alertness early in this optimum window (brown). Later use will affect sleep (caffeine half-life is about six hours). Optimum timing of exercise is not marked as it may vary depending on the optimum timing of zeitgebers (e.g. early morning for DSPS people and evening for ASPS people). For more details see: Biphasic nature of human sleep.
Figure: An exemplary interpretation of the two-process model of sleep regulation taken from an actual sleep log in SuperMemo for Windows. Aqua line represents circadian sleepiness. Green line represents homeostatic alertness (an inverse of the homeostatic sleep propensity). Red line represents overall alertness that is an inverse of overall sleep propensity. Best alertness is achieved when both components of sleepiness are at their lowest. Inspired by Alexander A. Borbély and Peter Achermann