biological processes that systematically vary over a 24-hour cycle are called
Circadian rhythm 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.[1]