Chills happen when your brain decides your body needs to be warmer and triggers rapid, involuntary muscle contractions to generate heat. This can be set off by cold environments, infections, strong emotions, or several medical conditions. The underlying mechanism is always the same: your hypothalamus, the brain’s built-in thermostat, detects a gap between your current body temperature and where it thinks your temperature should be, then fires off signals to close that gap.
How Your Brain Triggers Shivering
The process starts with temperature sensors in your skin. When these sensors detect cooling, they send signals to a region deep in the brain called the preoptic area. This region normally keeps shivering suppressed, but cold signals flip that switch off. Once that suppression lifts, a chain reaction moves through the hypothalamus and down into the brainstem, where motor neurons activate the skeletal muscles throughout your body.
Those muscles begin contracting and relaxing in rapid cycles, which is the shivering you feel. This is purely involuntary. You can’t will yourself to stop, because the signal bypasses conscious control entirely. The contractions generate heat as a byproduct, raising your core temperature. Your body also constricts blood vessels near the skin and triggers piloerection (goosebumps) to reduce heat loss at the surface. That’s why your skin looks pale and feels cool to the touch when you have chills.
In healthy adults, this shivering reflex typically kicks in when core body temperature drops below about 95°F (35°C), though mild chills can start before that threshold depending on how cold your skin is.
Why Fevers Make You Feel Cold
This is the part that confuses most people: if you have a fever and your body is already hotter than normal, why do you feel freezing? The answer is that an infection fundamentally changes what your brain considers “normal.”
When you’re fighting an infection, your immune cells release signaling molecules called pyrogens. These molecules cause the hypothalamus to produce prostaglandin E2, a chemical that raises the brain’s temperature set point. Instead of targeting the usual 98.6°F, your thermostat might now be set to 102°F or higher. Your actual body temperature hasn’t caught up yet, so your brain perceives a gap. As far as the hypothalamus is concerned, you’re too cold, even though you’re objectively warmer than usual.
The result is the classic fever sequence: first you get pale skin, goosebumps, and vasoconstriction as your body tries to conserve heat. Then shivering begins, sometimes intense enough to make your teeth chatter. You pile on blankets and curl up because your brain is genuinely signaling that you’re cold. Once the shivering raises your core temperature to match the new set point, the chills stop and you start feeling warm, sometimes uncomfortably so. When the infection clears and the set point drops back to normal, the process reverses: you sweat heavily as your body dumps the excess heat.
Emotional Chills and Music
Not all chills involve temperature regulation. You’ve probably felt a shiver run down your spine during a powerful piece of music, an emotional scene in a movie, or a moment of awe. Researchers call this “frisson,” and it’s a genuinely distinct phenomenon from thermoregulatory shivering.
A well-known study from McGill University measured brain activity during music that gave listeners chills and found that these moments trigger a release of dopamine, the same reward chemical involved in food, sex, and other pleasurable experiences. The researchers tracked changes in skin conductance, heart rate, breathing, and skin temperature that all correlated with how pleasurable listeners rated the music. Two separate brain circuits were involved: one connected to cognitive and motor systems (linked to anticipation, the buildup before a musical climax) and another tied to the limbic system, the emotional core of the brain.
What makes this remarkable is that music is abstract. It doesn’t directly help you survive or reproduce, yet it taps into some of the oldest reward circuitry in the human brain. The physical sensation of emotional chills, the tingling scalp, the raised arm hair, appears to be the autonomic nervous system responding to an intense emotional signal, borrowing the same hardware the body uses for temperature regulation but deploying it for a completely different purpose.
Chills After Surgery
If you’ve ever had general anesthesia, you may remember waking up shivering uncontrollably. This is one of the most common post-surgical complaints, and it has a straightforward explanation. Nearly all anesthetic drugs interfere with thermoregulation. They cause blood vessels to dilate, which allows warm blood from your core to flow outward to your limbs and skin, where it loses heat to the environment. Core body temperature can drop significantly in the first hour after anesthesia is administered.
At the same time, anesthetics raise the threshold at which your body would normally start shivering, meaning your temperature has to fall further before your brain reacts. Once the drugs begin wearing off, your thermoregulatory system snaps back online, detects that your core is too cold, and triggers intense shivering to compensate. Not every case of post-surgical shivering is caused by low temperature, though. Pain, stress, and heightened nervous system activity after an operation can also contribute.
Medical Conditions That Cause Chills Without Fever
Persistent or recurring chills without an obvious trigger can point to underlying health issues that affect how your body produces or regulates heat.
- Hypothyroidism. An underactive thyroid slows your metabolism, which means your body generates less heat at rest. Increased sensitivity to cold is one of the hallmark symptoms. In severe, untreated cases, hypothyroidism can progress to a dangerous state involving intense cold intolerance, extreme fatigue, and eventually loss of consciousness.
- Anemia. When your blood carries fewer red blood cells or less hemoglobin, less oxygen reaches your tissues. Since oxygen is needed to fuel the metabolic reactions that produce body heat, anemia can leave you feeling persistently cold, especially in your hands and feet.
- Low blood sugar. Hypoglycemia triggers a stress response that includes the release of adrenaline, which can cause shaking, sweating, and chills. This is your body’s alarm system signaling that it needs fuel.
- Anxiety and panic attacks. A surge of adrenaline during acute anxiety can cause the same constellation of symptoms: chills, trembling, and goosebumps. The body is mounting a fight-or-flight response even though there’s no physical threat.
Cold Exposure and the Shivering Threshold
The most straightforward reason for chills is simply being cold. Your skin doesn’t have to reach any dramatic temperature for the process to start. Cool air on exposed skin sends signals to the hypothalamus well before your core temperature drops, which is why you can shiver on a breezy evening even though your internal temperature is still perfectly normal. This is a preemptive defense: your brain is trying to prevent core cooling, not just respond to it.
Once your core temperature does begin to drop, shivering intensifies. The metabolic heat generated by maximal shivering can increase your energy expenditure several times over resting levels, which is why prolonged shivering leaves you exhausted. It’s essentially involuntary, full-body exercise. Below about 89.6°F (32°C), though, shivering paradoxically stops as the muscles lose their ability to contract effectively. This is a sign of moderate to severe hypothermia and a medical emergency.
Several factors influence how quickly you start shivering. People with more body fat have better insulation and tend to shiver later. Older adults and very young children have less effective thermoregulation and are more vulnerable to cold. Fatigue, dehydration, and alcohol consumption all impair your body’s ability to generate and conserve heat, lowering the threshold at which chills begin.