Why Does Seasonal Fatigue Feel Worse at Certain Times of the Year?
It arrives on a schedule. Not dramatically — just a gradual settling in of heaviness that feels different from ordinary tiredness. The alarm sounds at the same time it always does. The coffee is the same. The workload has not changed. But something about getting moving requires noticeably more effort than it did six weeks ago. Most people attribute this to temperature, shorter days, or reduced motivation. Those observations are not wrong. But they describe the surface of a process that runs considerably deeper — and understanding what is actually happening inside the body during seasonal transitions makes the experience considerably less confusing. The Circadian System Does Not Transition Instantly The human circadian clock — the internal timing system that regulates sleep, hormone release, metabolism, and body temperature across a 24-hour cycle — is calibrated primarily by light. Specifically, by the timing, intensity, and spectrum of light that enters the eyes each morning. When day ...