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Pacemaker cells in the hypothalamic suprachiasmatic nucleus governing circadian rhythm are arranged in two distinct networks, an evening network controlling sleep onset and a morning network controlling wake onset; both are primarily coupled to 24-hour solar days. In a study of 18 patients with rapid-cycling bipolar disorder, sleep and mood patterns oscillated with biweekly surges of the moon's gravitational tides, suggesting that one circadian pacemaker uncouples from solar rhythms and instead entrains onto 24.8-hour rhythms of lunar tidal days. These researchers examined possible links between clinical symptoms and phase shifts between the two pacemakers (e.g. 24-hour vs. 24.8-hour cycles) via one patient's mood and sleep records, which h…