Chills happen when your brain decides your body needs to be warmer and triggers your muscles to contract rapidly to generate heat. This can be caused by infections, hormonal changes, low blood sugar, emotional responses, or simply being in a cold environment. The underlying mechanism is the same in almost every case: a region deep in your brain called the hypothalamus acts as your internal thermostat, and when it senses (or is tricked into sensing) that your body temperature is too low, it fires off signals that make your muscles shiver.
How Your Body’s Thermostat Works
The hypothalamus sits at the base of your brain and constantly monitors your core temperature using signals from temperature sensors in your skin and internal organs. When it detects that you’re too cold, it responds in stages. First, it narrows blood vessels near the skin’s surface to keep warm blood closer to your vital organs. If that’s not enough, it triggers shivering, which is involuntary rapid contraction of your skeletal muscles. Those tiny contractions produce heat the same way exercise does, just without the voluntary movement.
This system works well when you’re actually cold. But several conditions can hijack it, making you shiver even when the room temperature is perfectly comfortable.
Fever and Infection
The most common reason for unexpected chills is that your body is fighting an infection. When your immune system detects bacteria or viruses, it releases signaling molecules that trigger the production of a compound called prostaglandin E2. This compound acts directly on the hypothalamus and raises your temperature set point, sometimes from the normal 98.6°F up to 101°F or higher.
Here’s what makes this feel so strange: your actual body temperature hasn’t caught up to the new, higher set point yet. So your brain interprets the gap the same way it would interpret being outside in the cold. It constricts your blood vessels and makes you shiver, all to generate heat and close that gap. This is why you can feel freezing and pile on blankets even though a thermometer shows you’re already running warm. Once your body temperature reaches the new set point, the chills usually stop, though you may then feel feverish and hot.
A temperature of 100.4°F or higher (measured orally, rectally, or with an ear thermometer) is considered a fever. Chills paired with a severe headache, stiff neck, or confusion warrant prompt medical attention, as these combinations can signal more serious infections.
Low Blood Sugar
When your blood sugar drops too low, your body releases stress hormones to try to bring it back up. This triggers a burst of activity in your nervous system that can cause shakiness, trembling, and a chill-like sensation. It’s not true shivering in the way a fever produces, but it feels similar. You might also notice a rapid heartbeat, sweating, or sudden anxiety alongside the shaking.
This is most common in people with diabetes who take insulin or other blood-sugar-lowering medications, but it can also happen if you’ve gone a long time without eating, exercised intensely on an empty stomach, or consumed a lot of alcohol without food. Eating something with carbohydrates and protein typically resolves the shakiness within 15 to 20 minutes.
Iron Deficiency and Anemia
If you feel cold and chilly more often than the people around you, low iron levels could be a factor. Iron-deficiency anemia reduces your body’s ability to carry oxygen efficiently in the blood, which impairs circulation. Poor circulation means less warm blood reaching your extremities, so your hands and feet feel cold first, and that constant chill can spread to the rest of your body. Fatigue, pale skin, and brittle nails are other common signs. A simple blood test can confirm whether your iron levels are low.
Thyroid Problems
Your thyroid gland plays a major role in regulating your metabolism and how much heat your body produces. When the thyroid is underactive, a condition called hypothyroidism, your metabolic rate slows down and your body generates less heat overall. Cold intolerance is one of the hallmark symptoms. People with hypothyroidism often describe feeling chilled in rooms that others find comfortable, needing extra layers, or having difficulty warming up after being outside. Other signs include unexplained weight gain, fatigue, dry skin, and sluggish digestion.
Hormonal Shifts During Perimenopause
Hot flashes get most of the attention during menopause, but cold flashes are a real and common experience. As estrogen levels fluctuate during perimenopause and menopause, the brain’s internal thermostat becomes more sensitive and less stable. This means your body can suddenly lose its ability to regulate temperature smoothly, causing rapid drops that feel like a wave of cold washing over you. These cold flashes are especially common at night, when your body temperature naturally dips slightly as part of your sleep cycle. The hormonal instability amplifies that normal dip into something much more noticeable.
Emotional and Musical Chills
Not all chills are medical. You’ve probably felt a shiver run down your spine during a powerful piece of music, a moving speech, or a moment of intense fear. These “frisson” experiences are driven by your autonomic nervous system, the same system that controls your fight-or-flight response. When you hear music with sudden dynamic changes, unexpected harmonies, or a soaring vocal line, your nervous system spikes in arousal. Your heart rate increases, your skin conductance jumps, and goosebumps appear.
What makes this especially interesting is that the experience activates your brain’s dopamine reward system in two distinct phases. The anticipation of an emotional peak lights up one region, and the moment of release lights up another. This is the same reward circuitry involved in food, sex, and addictive substances. Your brain essentially learns to crave the music that gives you chills, creating a mild form of pleasurable anticipation each time you hear it. These chills are completely harmless and, for many people, one of the most enjoyable physical sensations music can produce.
Medication Side Effects
Several types of medications can cause chills or shivering as a side effect. This is most notable when drugs that affect serotonin levels are combined or taken at high doses. Antidepressants, certain migraine medications, opioid painkillers, and even over-the-counter cough medicines containing dextromethorphan can all raise serotonin activity in the brain. When serotonin levels climb too high, a condition called serotonin syndrome can develop, with shivering and goosebumps among the early symptoms. Other signs include agitation, rapid heart rate, and muscle twitching.
If you’ve recently started a new medication or combined two medications and notice unexplained chills along with restlessness or muscle stiffness, that’s worth bringing up with your prescriber quickly. Serotonin syndrome can escalate from mild to serious.
Simple Cold Exposure and Sleep Deprivation
Sometimes the answer is straightforward. If you’re underdressed for the weather, sitting in a drafty room, or wet from rain or sweat, your skin’s temperature receptors send cooling signals to the hypothalamus and it responds with shivering. This is normal thermoregulation working exactly as designed.
Sleep deprivation can also make you more susceptible to chills. Poor sleep disrupts your body’s temperature regulation, and chronic sleep loss has been linked to slightly lower core body temperatures during waking hours. If you’ve been sleeping poorly and notice you’re colder than usual, the two may be connected.
Patterns Worth Paying Attention To
Occasional chills after cold exposure or during a brief illness are nothing unusual. What matters more is the pattern. Chills that come and go for weeks without an obvious cause, chills paired with unexplained weight changes or fatigue, or chills accompanied by a fever above 103°F all point toward something that deserves investigation. Chills with a stiff neck, confusion, or sensitivity to light are more urgent and shouldn’t wait for a routine appointment.
For most people, though, chills are a temporary and self-limiting signal that your body is doing exactly what it’s supposed to do: generating heat when it thinks you need it.