The Emergence of Circadian Rhythms Consistent daily variations in movement, heart rate, and other variables are evident during the fifth month of gestation in the human fetus. After birth, the synchronization of infants’ circadian rhythms to a day–night cycle usually occurs by 2 or 3 months of age (Mistlberger & Rusak, 2005). Daytime exposure to bright light helps establish these regular rest–activity circadian rhythms.

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