A circadian rhythm
The term circadian comes from the
Although circadian rhythms are endogenous ("built-in", self-sustained), they are adjusted (entrained) to the local environment by external cues called
In 2017, the
The earliest recorded account of a circadian process dates from the 4th century B.C.E., when
The first recorded observation of an endogenous
In 1896, Patrick and Gilbert observed that during a prolonged period of sleep deprivation, sleepiness increases and decreases with a period of approximately 24 hours.
 In 1918, J.S. Szymanski showed that animals are capable of maintaining 24-hour activity patterns in the absence of external cues such as light and changes in temperature.
 In the early 20th century, circadian rhythms were noticed in the rhythmic feeding times of bees. Extensive experiments were done by