Why Do Circadian Rhythms Exist?

Circadian rhythms have evolved as a result of adapting to the Earth's consistent 24-hour rotation and its 23.5-degree tilt while orbiting the Sun.These environmental conditions create daily and seasonal cycles, necessitating life forms to adapt.From early microorganisms to complex vertebrates, all life on Earth has developed genes that govern circadian rhythms to prepare for day-night changes.For early microorganisms, sudden exposure to UV light during the day posed a threat, prompting the evolution of mechanisms to predict and prepare for these changes, resulting in circadian rhythms [1].However, humanity first realized the existence of circadian rhythms in the 18th century when French scientist Jean Jacques d'Ortous de Mairan observed that mimosa plant leaves opened and closed in a 24-hour cycle, even in constant darkness [2].This suggested an internal biological clock.In 1971, Konopka and Benzer [3] discovered a gene in fruit flies, called "period, " that influenced circadian rhythms.Later, Jeffrey C. Hall, Michael Rosbash, and Michael Young elucidated the mechanisms, discovering that the period gene and timeless gene work together to create a circadian rhythm matching the 24-hour day-night cycle [2].Research on fruit flies revealed that the period gene's expression starts during the day, producing PER protein in the cytoplasm.At night, PER moves into the nucleus, inhibiting the period gene's activity and creating a 24-hour cycle [4].The TIM protein, produced by the timeless gene, binds with the PER protein, enabling its entry into the nucleus.Consequently, the PER protein inside the nucleus can inhibit the operation of the period gene.Furthermore, a protein produced by another gene named doubletime, DBT, ensures that the PER protein does not accumulate in the cy-