Why Do We Feel Cold When We Have a Fever?
A fever usually announces itself in a strange way: you feel freezing. You pile on blankets while your temperature is climbing, then wake up sweating hours later. Both phases are the same system doing its job, a thermostat in the brain being deliberately reset.
The thermostat gets a new setting
Your core temperature is managed by the hypothalamus, which holds it near 37 °C in health. During an infection, immune signalling molecules called pyrogens reach the brain and trigger production of prostaglandin E2, which acts on the preoptic area of the hypothalamus. The effect is a higher target temperature: the body's set point is turned up, sometimes to 39 °C or more. A review in Critical Care describes how this prostaglandin signal slows the firing of warm-sensitive neurons, which is what starts the rise.
Feeling cold is the heating phase
Once the set point jumps, your actual temperature is suddenly too low relative to the new target. The body responds exactly as it would in cold weather: blood vessels in the skin tighten to conserve heat, which is why you look pale and feel cold to the touch, and muscles start shivering to generate warmth. These chills are not a separate illness. They are the body's heating system running hard until the core temperature matches the raised setting.
Sweating means the setting dropped
When the infection subsides or a fever medicine blocks prostaglandin production, the set point falls back to normal. Now the body overshoots: your temperature is higher than the target, so the cooling system switches on. Blood vessels open, skin flushes, and you sweat. That sweaty, too-hot phase is the mirror image of the chills, the same thermostat returning to its usual number.
Why the body does this at all
Fever is one of the oldest defence responses in the animal kingdom, and the raised temperature is thought to help the host response fight infection, which is why mild fevers are often left to run their course. The chills and the sweats are uncomfortable side effects of the body deliberately moving its temperature, first up and then back down, around a target the brain itself chose.
Facts checked: 22 October 2026
Sources: Walter, E. J., Hanna-Jumma, S., Carraretto, M., and Forni, L. (2016). The pathophysiological basis and consequences of fever. Critical Care. PMC4944485.