Cold weather can dull your thirst by up to 40%, so you can become dehydrated this fall and winter without ever feeling thirsty or noticing it. Sweat under layers, dry indoor heat and every cold breath still drain your body's fluids, and the fatigue, headaches and dizziness that follow are easy to chalk up to winter blues. Here's why your body won't tell you, and how to stay hydrated when temperatures drop.