Bipolar disorders have an onset in late adolescence or early adulthood and patients may experience alternating episodes of mania and depression, with euthymic periods interspersed between these extremes of mood. Clinical research studies have shown that bipolar disorder patients exhibit disruptions in circadian and seasonal rhythms, even when they are symptom free. In addition, some bipolar patients display pronounced seasonal patterns in occurrence of manic and depressive episodes, time of year for disease onset, and age of onset. Several groups have emphasized the impact of seasonal changes in sunlight intensity on bipolar disorder, especially in locations farther from the equator. In this paper, we examine rate of change of solar insolation during the spring and fall in locations that vary in their distance from the equator and propose that seasonal changes in sunlight intensity may be tracked by the suprachiasmatic nucleus and affect disease onset and progression in seasonally susceptible bipolar patients.Copyright © 2020. Published by Elsevier Ltd.