Chronobiology

Earth’s rotation created predictable light-dark cycles and organisms evolved to anticipate them. Internal time-keeping and organization around external zeitgebers is an evolutionary advantage for maintaining energy efficiency, coordinating DNA repair and synthesis, and optimizing basically any cellular function. Because the day-night cycle is so stable, the advantage of molecular clocks has never disappeared. Evidence suggests that circadian rhythms evolved independently in many lineages but they all appear to rely on similar principles.^[1,2]

Nearly every cell in the human body has a circadian clock.^[3] This includes not only normal tissues but also cancers that can have stable or dysregulated rhythms.^[4] Circadian dysregulation may even be a key driver of carcinogenesis in some cancer types.^[5] On the molecular level, circadian rhythms are organized through transcription-translation feedback loops that create endogenous oscillators.^[6] In humans, these loops involve the gene families CLOCK, BMAL, PER, and CRY. Peripheral clocks are coordinated by the master circadian pacemaker in the suprachiasmatic nucleus (SCN) in the brain. The SCN synchronizes to the solar day, with light serving as the main external zeitgeber (time cue).