You’re bundled under layers, teeth chattering, convinced you’re freezing — only to discover your temperature is climbing well above normal.
It doesn’t make sense at first glance, but your body is reacting exactly as it’s supposed to.
The reason lies deep in the brain’s internal thermostat, which doesn’t care how you feel — it’s focused on survival.
The moment it decides your body needs to heat up to fight off a threat, it flips a biological switch. From that moment, your body no longer thinks 37°C is “normal.” It now wants more.
As a result, your system behaves as though it’s in a cold environment. You start to shiver, your skin cools down, and you may even get goosebumps.
In truth, you’re not cooling down — you’re actively heating up from the inside out.
Your Brain’s Thermostat Takes Over
At the heart of this process is the hypothalamus — the part of your brain that manages internal body temperature. When your immune system detects an invader, it sends chemical messengers (known as pyrogens) that tell the hypothalamus to raise the temperature.
This temperature shift sets off a chain reaction:
- Blood vessels in the skin constrict to retain heat
- Muscles begin to contract, causing shivering
- The body ramps up heat production
These responses are identical to what happens when you’re truly cold — which is why you experience that strange sensation of freezing while your fever rises.
Once your temperature stabilizes, the chills fade. And when your fever starts to drop? That’s when the sweating kicks in.
When Fever Means Trouble
While most fevers are harmless and even helpful, they can occasionally signal something more serious.
The most common triggers include viral infections like the flu or common cold, bacterial issues such as throat or bladder infections, and respiratory illnesses like pneumonia.
Common signs your fever is spiking include chills, muscle aches, headaches, fatigue, loss of appetite, and eventually sweating as the body cools down.
In rare cases, very high fevers (above 40°C) or symptoms like confusion, neck stiffness, or unresponsiveness require immediate medical attention.
The key takeaway: That sudden chill when your fever starts rising isn’t a sign you’re getting worse — it’s your body doing exactly what it needs to fight back.
This article is based on information from Illustreret Videnskab