Autumn fatigue has a biological basis: how seasonal light loss affects mood and sleep

Each autumn, as daylight retreats and temperatures fall, millions of people feel a heaviness descend that is neither weakness nor imagination — it is biology. The human body, calibrated by millennia of solar rhythms, struggles to reconcile shortening days with an internal clock that cannot adapt overnight, producing disruptions in serotonin, melatonin, and cortisol that touch mood, sleep, pain, and motivation alike. Science now offers both an explanation and a path forward: understanding the seasonal body is the first step toward moving through the darker months with greater ease.