Muggy Weather and Fatigue: The Physiology
Muggy weather makes you tired because humid air blocks your main cooling system, and your body compensates in ways that are metabolically expensive. When sweat cannot evaporate, your heart rate rises, blood is diverted to the skin, plasma volume falls, and core temperature drifts upward. That cardiovascular work happens whether or not you are exercising. Add disrupted sleep from warm nights and mild dehydration from fluid loss without cooling benefit, and the fatigue is not psychological — it is the predictable output of a body running a continuous background load it does not run on a dry day.
The subjective experience is distinctive: heavy limbs, poor concentration, a reluctance to move, and a sense that ordinary tasks cost more than they should. Nothing about that is imaginary.