The clock begins with environmental cues (Zeitgebers) such as light, food, temperature, exercise, electronics, or social interactions. These signals program/align the “master clock” found in the SCN, which then signals the peripheral body clocks. The molecular clock pathway is shown in the inset, with the primary and secondary feedback loops. The gene-protein interactions create an oscillating rhythm of clock-controlled gene expression based on the time of day. A subset of these controlled genes then reintegrates into the feedback system.