Fevers make you feel cold because your brain temporarily resets what it considers “normal” body temperature to a higher number. When that happens, your actual body temperature is suddenly below the new target, and your body reacts the same way it would if you walked outside in winter: it constricts blood vessels, triggers shivering, and makes you reach for a blanket. You’re not actually cold. Your brain just thinks you are.
Your Brain’s Internal Thermostat
Deep inside your brain, a region called the hypothalamus works like a thermostat. Under normal conditions, it keeps your core temperature hovering around 98.6°F (37°C). When you get an infection, your immune system releases signaling molecules that travel to the hypothalamus and essentially turn the dial up, sometimes to 101, 102, or higher.
Here’s what happens at the molecular level. Bacteria, viruses, or other invaders trigger your immune cells to release inflammatory signals. These signals cause the hypothalamus to produce a chemical called prostaglandin E2, which acts directly on temperature-regulating neurons. The result: your brain’s set point jumps. A core temperature of 98.6°F that felt perfectly fine five minutes ago now registers as “too cold” by your brain’s new standard. Your body launches into warming mode, and you feel freezing.
Why You Shiver and Feel Freezing
Once the set point rises, your body uses the same cold-defense strategies it would use on a frigid day. Blood vessels near your skin’s surface narrow, pulling warm blood away from the surface and toward your core. That’s why your hands, feet, and skin may feel icy to the touch. With less warm blood flowing near the surface, your skin literally is cooler, which reinforces the sensation of being cold.
Then comes shivering. Your muscles contract rapidly and involuntarily, generating heat through the sheer inefficiency of the process. Shivering can be remarkably effective at producing warmth. These repeated skeletal muscle contractions burn energy and release heat, steadily pushing your core temperature up toward the hypothalamus’s new target. You might also curl up, pull on layers, or burrow under blankets, all behavioral responses your brain drives to close the gap between your current temperature and the new set point.
This phase, where your temperature is climbing but hasn’t yet reached the new set point, is when you feel the coldest. It can last anywhere from minutes to over an hour depending on how high and how fast the set point rises.
Chills vs. Rigors
Most fevers come with a general sensation of chilliness, and that’s normal regardless of the cause. But sometimes the shaking is far more intense: teeth chattering, the whole bed shaking, muscles clenching hard. These episodes are called rigors, and they signal a particularly rapid spike in the set point.
A mild sense of being chilly is common with virtually any fever and doesn’t point to a specific cause. Rigors, on the other hand, especially one or two sudden, violent episodes of shaking, are more characteristic of acute bacterial infections. The distinction matters because the intensity of the chill can give your doctor useful information about what’s causing the fever.
What Happens When the Fever Breaks
Eventually, your immune system gets the upper hand, or you take a fever reducer, and the hypothalamus lowers the set point back to normal. Now the opposite problem occurs: your core temperature is 102°F, but your brain’s thermostat just dropped back to 98.6°F. Suddenly you’re “too hot” by the brain’s new math.
Your body flips into cooling mode. Blood vessels near the skin open wide, flushing your skin with warm blood. Sweat glands kick in. You go from bundled under blankets and shivering to throwing off covers and feeling drenched. This is the defervescence phase, and it’s the mirror image of what made you feel cold in the first place. The sweating and flushed skin are your body dumping the excess heat it worked so hard to build up.
Should You Bundle Up or Cool Down?
When you’re in the shivering phase and feel cold, your instinct is to pile on blankets. That’s generally fine and will make you more comfortable. Research on cooling blankets in hospitalized fever patients found that warmer blanket temperatures provided similar cooling rates as colder ones while being significantly more comfortable and causing less shivering. In other words, fighting someone’s chill with aggressive cooling doesn’t speed up recovery and just makes them miserable.
That said, you don’t want to overshoot. Wrapping yourself in heavy insulation won’t dangerously raise your temperature in most cases, but it can make you less aware of how hot you’re getting, particularly in young children. A light blanket during the chill phase and removing layers once you start sweating is a practical approach.
Fever reducers like acetaminophen and ibuprofen work by blocking the production of prostaglandin E2, the same chemical that raised the set point in the first place. By lowering the set point back toward normal, these medications reduce both the fever itself and the uncomfortable chills that come with it. They don’t fight the underlying infection, but they do interrupt the cycle that makes you feel cold.
Why Fever Exists in the First Place
Feeling cold during a fever is unpleasant, but it’s not a malfunction. Fever is an evolved immune response. Many pathogens reproduce more slowly at higher temperatures, and several immune functions work more efficiently when the body is warmer. Your brain deliberately raises the thermostat because, for most infections, a hotter body is a more hostile environment for whatever is making you sick. The chills are simply the side effect of your body racing to reach that new, strategically higher temperature.