Circadian rhythm
![Some features of the human circadian (24-hour) biological clock](/Images/godic/202501/27/Biological_clock_human.svg3900.png")
![Illustration of the morning (yellow) and evening (gray) circadian clock loops in Arabidopsis, entrainable through light. Transcriptional regulation is shown through black lines and protein complexes are denoted by dashed black lines. Post-translational regulation is shown by dashed red lines. Light sensitive elements are denoted with lightning bolts and yellow circles.](/Images/godic/202501/27/TOC1_interactions3900.jpg")
![A variation of an eskinogram illustrating the influence of light and darkness on circadian rhythms and related physiology and behaviour through the suprachiasmatic nucleus in humans.](/Images/godic/202501/27/Circadian_rhythm_labeled3900.jpg")
![A short nap during the day does not affect circadian rhythms.](/Images/godic/202501/27/Day_Sleepers_crop3900.jpg")
A circadian rhythm /sɜːrˈkeɪdiən/ is any biological process that displays an endogenous, entrainable oscillation of about 24 hours. These 24-hour rhythms are driven by a circadian clock, and they have been widely observed in plants, animals, fungi, and cyanobacteria.