Chills during a fever happen because your brain has temporarily raised its target temperature, making your normal body heat feel too cold. It’s the same sensation you’d get stepping outside on a winter day, except the “cold” is coming from inside: your body thinks it needs to be warmer than it currently is, so it triggers shivering and other heat-generating responses to close the gap.
How Your Brain Resets Its Thermostat
Your brain has a built-in thermostat located in a region called the hypothalamus. Under normal conditions, it keeps your core temperature hovering around 98.6°F (37°C). When you get an infection, the process of creating a fever begins not with heat, but with a signal to change the target.
Here’s the sequence: bacteria, viruses, or other invaders trigger your immune cells to release chemical messengers (including ones called IL-1, IL-6, and TNF). These messengers travel to the hypothalamus and cause it to produce a compound called prostaglandin E2. That compound is the key player. It acts on specific neurons in the thermostat and pushes the set point upward, sometimes to 101°F, 102°F, or higher.
Once the set point rises, your brain perceives a problem. Your actual body temperature is still 98.6°F, but the thermostat now “wants” 102°F. As far as your hypothalamus is concerned, you’re running several degrees too cold. So it launches the same warming strategies it would use if you were standing in a snowstorm.
Why Shivering Produces Heat
Shivering is your body’s most powerful short-term heating tool. It works through rapid, involuntary contractions of skeletal muscles. These contractions burn energy, and most of that energy is released as heat rather than movement. In animal studies, the chemical signal that raises the fever set point can increase muscle activity by more than 1,000% above baseline levels, which gives you a sense of how aggressively the body ramps up heat production.
At the same time, your body conserves the heat it already has. Blood vessels near the surface of your skin constrict, pulling warm blood away from the skin and toward your core. This is why your hands and feet may feel icy, your skin might look pale, and you feel an overwhelming urge to curl up under blankets. Tiny muscles at the base of your hair follicles also contract, causing goosebumps. In animals with fur, this traps an insulating layer of air. In humans it doesn’t do much, but the reflex remains.
Together, shivering and vasoconstriction push your actual temperature upward until it matches the new, higher set point. Once it does, the chills stop. This is why a fever often follows a predictable pattern: chills first, then a plateau of feeling hot, then sweating as the set point eventually drops back to normal and your body needs to shed excess heat.
The Three Phases of a Fever
Understanding the phases helps explain why your symptoms shift over time.
In the rising phase, the set point has gone up but your body hasn’t caught up yet. You feel cold, shiver, pile on blankets, and may notice pale skin and a faster heart rate. Your temperature is climbing but hasn’t peaked.
In the plateau phase, your core temperature has reached the new set point. The chills fade. You may feel hot, flushed, and uncomfortable, but the shivering stops because your body no longer detects a gap between actual and target temperature.
In the breaking phase, the immune response winds down and the hypothalamus lowers the set point back toward normal. Now your body is “too hot” relative to the thermostat, so it does the opposite of what caused the chills: blood vessels in your skin dilate, and you start sweating. This is when people say the fever “broke.” You may feel drenched but relieved.
What Counts as a Fever
Most healthcare providers define a fever as an oral temperature at or above 100.4°F (38°C). Temperatures between 99.5°F and 100.3°F are generally considered a low-grade fever. In adults, fevers below 103°F (39.4°C) are typically not dangerous on their own. Untreated fevers above 105.8°F (41°C) can become dangerous and need medical attention.
What to Do When You’re Shivering With a Fever
The instinct to bundle up during chills is reasonable. Layering blankets and warm clothing can ease the discomfort of the shivering phase. A warm drink helps too. Your body is going to raise its temperature regardless, so there’s no harm in making yourself comfortable while it does.
Once the chills pass and you start feeling hot, switch to lighter clothing and stay hydrated. Fever increases fluid loss through sweating and faster breathing, so water and electrolyte-containing drinks matter more than usual. Over-the-counter fever reducers work by lowering the hypothalamic set point back toward normal, which is why they can bring both the temperature and the discomfort down at the same time.
Chills That Need Urgent Attention
Most fevers with chills are caused by common infections and resolve on their own. But certain accompanying symptoms suggest something more serious. Seek immediate medical attention if chills and fever come with any of the following: a stiff neck (especially pain when bending your head forward), mental confusion or altered speech, a rash, unusual sensitivity to bright light, persistent vomiting, difficulty breathing or chest pain, abdominal pain, pain when urinating, or seizures. These combinations can point to conditions like meningitis, sepsis, or other infections that need rapid treatment.