Circadian rhythms are cyclic and persistent patterns of behavior, physical changes, and mental characteristics exhibited by most life on Earth, from the smallest bacteria to the largest redwood tree. These rhythms roughly follow 24 hour periods, reflecting the amount of time it takes for the Earth to complete a rotation. The study of circadian rhythms and the internal clocks which most creatures seem to have is known as chronobiology. Researchers study circadian rhythms to learn more about life on Earth, and how to treat various conditions such as sleep disorders.

Several characteristics distinguish circadian rhythms. The first is that these changes will be retained through dramatic changes in environmental conditions. For example, an animal in the dark will still manifest periods of increased and decreased activity which correspond with a 24 hour cycle. Repeated input from external stimuli can also reset circadian rhythms, as anyone who has switched time zones is probably aware. In addition, fluctuations in temperature do not appear to impact circadian rhythms.

The term was coined by Franz Halberg, a researcher at the University of Minnesota. Halberg was fascinated by the cyclical patterns of behavior which could be observed in things like plants, which actually slowly move over the course of a day to take advantage of changing light conditions. The word is derived from the Latin words circa, meaning “around,” and dies, for “day.” The study of circadian rhythms links a number of disciplines including chemistry, general biology, genetics, physiology, and even psychology. Halberg is widely regarded as the father of chronobiology, although circadian rhythms have been observed and described since the 1700s.

Humans tend to be most interested in circadian rhythms in terms of how they affect sleep patterns. When humans experience sleep disorders such as insomnia, these problems can sometimes be linked to an imbalance in circadian rhythms which could potentially be fixed. Circadian rhythms also explain why people experience periods of more alertness at certain times of the day, and they dictate when humans feel sleepy or hungry as well.

A clear genetic link to circadian rhythms has been established by researchers, who suggest that these very basic patterns have probably been on Earth almost as long as living organisms have. Primitive bacteria demonstrate circadian patterns, for example. Animals with brains also clearly have an internal biological clock which dictates circadian patterns.