there is an endogenous "clock" in all kinds of organisms, which controls the rhythm of various behaviors and physiological functions: when to sleep, when to wake up, when to secrete hormones. Animals, plants and even single-celled organisms have such a circadian rhythm that even if there is no external signal of day and night, the "clock" in the body will still run regularly, and each cycle is close to 24 hours.

but not all creatures live at this pace. Recently, researchers have discovered an "endogenous clock" with a strange rhythm: in some spiders, their "clock setting" is surprisingly fast.

the researchers placed spiders in a completely dark environment and detected their activity patterns with infrared light. It was found that the endogenous cycles of the three Araneae species were far less than 24 hours, and the shortest one was only 17.4 hours on average, which is also the shortest known record.

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https://www.newscientist.com/article/2153407-spiders-reset-body-clocks-to-avoid-5-hour-jet-lag-every-day/

another little knowledge about circadian rhythm: Today I Learned: sleep a little late every day