cues such as light and changes in temperature.

In the early 20th century, circadian rhythms were noticed in the rhythmic feeding times of bees. Auguste Forel, Ingeborg Beling, and Oskar Wahl conducted numerous experiments to determine whether this rhythm was attributable to an endogenous clock. The existence of circadian rhythm was independently discovered in fruit flies in 1935 by two German zoologists, Hans Kalmus