<p>Based on several lines of evidence, we hypothesize that rapid cycling in a circular, lunar-synchronized type of bipolar disorder is a pathological response to lunar cycles that occur physiologically in normal human sleep. The hypothesis is based on causal implications of observations that 1) reductions in sleep precede onsets of mania during the natural course of bipolar disorder, 2) experimental sleep deprivation can precipitate onsets of mania in depressed patients with circular, rapid cycling, 3) the duration of normal human sleep decreases, and its onset is delayed during the approach of full moons and new moons, and 4) onsets of mania in patients with circular rapid cycling most often occurred near times of full moons and/or new moons. To complete the evidence, it remained to be shown that circular, rapid cycling patients experience lunar cycles in sleep similar to those that occur in healthy individuals. To address that question and provide proof of concept for the hypothesis, we analyzed normalized full moon-to-new moon and new-moon-to full moon segments of three patients’ longitudinal records of sleep and found that waveforms in sleep onset and duration were similar to those in healthy individuals.</p>