- Physiology. an innate periodicity in an organism’s physiological processes, as sleep and wake cycles.
- a cyclically recurring pattern of physiological states in an organism or organ, such as alpha rhythm or circadian rhythm; believed by some to affect physical and mental states and behaviour
- A biologically inherent cyclic variation or recurrence of an event or state, such as sleep cycles and circadian rhythms.
- A recurring biological process, such as sleep, that is controlled by the circadian rhythms of an organism. See more at circadian rhythm.