A circadian rhythm or circadian cycle is a natural, internal process that regulates sleep–wake cycle. These rhythms are 24-hour cycles that are part of the body’s internal clock. They run in the background to carry out essential functions and processes. This circle repeats roughly every 24 hours. It can refer to any process that originates within an organism and responds to the environment. These 24-hour rhythms have been widely observed in animals, plants, fungi and cyanobacteria. One of the most important and well-known circadian rhythms is the sleep-wake cycle.
Different systems of the body follow circadian rhythms that are synchronized with a master clock in the brain. This master clock is directly influenced by environmental cues, especially light, which is why circadian rhythms are tied to the cycle of day and night.
When properly aligned, a circadian rhythm can promote consistent and restorative sleep. But when this circadian rhythm is thrown off, it can create significant sleeping problems, including insomnia. Research is also revealing that circadian rhythms play an integral role in diverse aspects of physical and mental health.
The term circadian comes from the Latin circa, meaning “approximately”, and dies, meaning “day”. Processes with 24-hour cycles are more generally called diurnal rhythms. Diurnal rhythms should not be called circadian rhythms unless they can be confirmed as endogenous, and not environmental.
Although circadian rhythms are endogenous, they are adjusted to the local environment by external cues called zeitgebers. Some of these external cues include light, temperature and redox cycles.