to daily stimuli. He noted that 24-hour patterns in the movement of the leaves of the plant Mimosa pudica persisted, even when the plants were kept in constant darkness.

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