A circadian rhythm is a rhythm that repeats about every 24 hours. Plants and animals have these built-in cycles which allow them to flower at the right time, sleep at the right time etc. The word "circadian" comes from the Latin "circa", "around", and "diem" or "dies", "day", meaning "about a day." The study of biological time-keeping rhythms such as daily, tidal, weekly, seasonal, and annual rhythms, is called chronobiology.