Circadian cycle is the cycle of physiological changes in the body in the course of a single day. Undisrupted circadian cycle is usually a bit longer than 24 hours. Hence the name circa-.
Figure: Changes in alertness in the course of a 24-hour circadian cycle. Alertness is here expressed 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 time, i.e. the number of hours since waking. Vertical axis corresponds with average recall in learning based on spaced repetition. Blue dots express recall at a given circadian time. Thick blue line is the approximation of circadian alertness derived from a two-component model of sleep (inspired by Alexander Borbely). Learning performance (thinner line) provides a good match to circadian alertness (thicker line).