Circadian phase

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Circadian phase, circadian time (ct) or circadian frame is the difference between the clock time and the expected natural waking time. For example, Phase 3 or ct 3 means "3 hours from natural waking". If we label a specific time with a circadian phase number, it usually means the number of hours that pass between the end of the subjective night and that reference time. Optimum time for siesta occurs between the circadian Phase 7 and Phase 8. This means that we should nap 7-8 hours from natural waking. See also: Sleep phase.

I often speak of perfect circadian alignment, which is the natural alignment of sleep with the demands of the body clock. In terms of circadian phase, perfect alignment occurs when we naturally wake up at Phase 0. Perfect circadian alignment is conducive to achieving maximum circadian amplitude, which can be a good marker of circadian health. Circadian health in modern society is very poor. It leads to sleep disorders, obesity, depression, diabetes, metabolic disorders, and many more.

Warning: Circadian phase should not be confused with sleep stage such as REM, wake time (measured from awakening even if it is not natural), or zeitgeber time such as ZT4, zt 6, etc.. Zeitgeber time counts hours from the point of a zeitgeber (e.g. lights on).

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).

Optimizing the timing of brainwork with respect to the circadian cycle
Optimizing the timing of brainwork with respect to the circadian cycle

Figure: Optimizing the timing of brainwork with respect to the circadian cycle. This 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 sleep cycle period may be longer than 24 hours). Light blue dots 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 the 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 a red peak. Sleep maintenance circadian component correlates (1) negatively with core body temperature, ACTH, cortisol, catecholamines, and (2) positively with: melatonin and REM sleep propensity. Optimum timing of brainwork requires both (1) low homeostatic sleepiness, and (2) low circadian sleepiness. There are two high quality alertness blocks during the day: the first after the awakening, and the second after the siesta. Both blocks are marked as yellow bands below the graph. For best learning, and for best creativity, use these two yellow blocks of time. Caffeine can only be used to enhance alertness early in the optimum brainwork 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

Optimizing the timing of meals (example)
Optimizing the timing of meals (example)

Figure: Example of optimized meal timing inspired by Mediterranean habits. One major meal comes in Phase 8, and directly precedes the siesta. Evening snack comes in Phase 16 (or earlier). In terms of intermittent fasting, in the presented example, the period without meals spans 16.5 hours. Important: each person's circadian cycle may differ in terms of phase, period and amplitude. The timing in the graph is exemplary and cannot be followed without a study of individual sleep data, health, metabolic profile, lifestyle, latitude, climate, exercise regimen, and many more. For example, for people who need more hours for night sleep, circadian phase may be lower. It is more important to make sure that last meal precedes sleep by no less than 2-4 hours. Sleep data derived from SleepChart. The chart is based on: Optimizing the timing of brainwork

Figure: Circadian graph in a biphasic sleeper. All sleep blocks are plotted in reference to the predicted optimum waking hour. The red line shows the average length of sleep blocks initiated at a given circadian hour (time from arising). The blue line shows the frequency of sleep initiated at that circadian hour. The optimum non-shifting length of the waking day is determined by the crossing of the green and red lines. In this case it is 20 hours (this value usually drops after block consolidation and is then more accurate). The most frequently used length of the day is also 20 hours indicating regular adherence to the optimum. Optimum siesta time from waking is likely indicated by siesta frequency that peaks at 9 hours. Usual siesta length is 1.2 hour and is nearly independent of the circadian hour. Maximum unconsolidated nighttime sleep occurs when initiated 17 hours from waking. The optimum unconsolidated length is less than 4 hours