Acts as an internal timer and makes animals nocturnal or diurnal innate behavior
survival beahior

biological clock

learned behavior

Circadian rhythm, or circadian clock, is an internal biological clock that regulates sleeping and feeding patterns, as well as a whole host of other biological functions in all living organisms. Organisms most active in the daytime are diurnal, humans included, whereas nocturnal organisms come alive at night.

gregor could have been bullying her