Chills happen when your brain decides your body needs to be warmer, whether that’s because you’re actually cold, fighting an infection, or experiencing a powerful emotion. The sensation comes from rapid, involuntary muscle contractions that generate heat, controlled by a thermostat-like region in your brain called the hypothalamus. But not all chills work the same way, and the reason behind yours depends on what’s triggering them.
How Your Brain Triggers Chills
Your hypothalamus sits at the base of your brain and functions as a temperature control center. It constantly receives signals from temperature sensors in your skin and throughout your body. When those signals indicate you’re too cold, or when internal chemistry shifts the target temperature upward (as in a fever), the hypothalamus sends commands through your nervous system to your skeletal muscles.
Those muscles begin contracting rapidly and involuntarily, producing the shivering you recognize as chills. This process, called shivering thermogenesis, is remarkably effective at generating heat. It’s the same muscle system you use for voluntary movement, but during chills, the thermoregulatory center hijacks it to produce a repetitive, stereotyped motor pattern you can’t consciously stop. The hypothalamus also triggers other warming responses at the same time, like constricting blood vessels near your skin to keep warm blood closer to your core.
Fever: When Your Thermostat Resets
This is the most common reason people experience chills that seem to come out of nowhere. When your immune system detects an infection, immune cells release small molecules called pyrogens into your bloodstream. These pyrogens reach the hypothalamus and essentially raise its temperature set point, sometimes by several degrees.
Here’s the key detail most people miss: you feel cold and start shivering not because your body temperature has dropped, but because your brain now thinks your normal 98.6°F is too low. Your hypothalamus is acting as though the “correct” temperature is, say, 102°F, so it treats your current body temperature like you’ve been standing in the cold. It fires up the same shivering response it would use on a freezing day. That’s why chills often hit before you realize you have a fever. Once your body temperature climbs to match the new set point, the chills stop and you feel hot instead.
Cold Exposure and Hypothermia
The most straightforward cause of chills is simply being cold. When your core body temperature dips below its normal range, the hypothalamus activates shivering to generate heat. In mild hypothermia, when core temperature falls between 90°F and 95°F, shivering is vigorous and continuous. This is your body working hard to warm itself back up.
What’s important to know is that shivering has limits. If your core temperature drops further, into the moderate hypothermia range below about 90°F, shivering stops entirely. Your muscles are too exhausted and your metabolic processes too sluggish to sustain it. The absence of shivering in someone who is clearly very cold is a serious warning sign, not a sign of improvement.
Emotional and Aesthetic Chills
You’ve probably felt chills run down your spine during a powerful piece of music, an inspiring speech, or an emotionally intense moment in a film. These are real, measurable physiological events sometimes called “frisson” or aesthetic chills, and they have nothing to do with temperature regulation.
Instead, these chills are driven by your brain’s reward system. When you experience something deeply moving, your brain releases dopamine in the same reward circuits activated by food, sex, and certain drugs. Brain imaging studies show a pattern of activity during aesthetic chills that closely resembles euphoria, with increased signaling in reward-processing areas and a simultaneous quieting of the brain’s fear center. The result is goosebumps, a tingling sensation, and sometimes a wave of warmth or electricity along your skin. Not everyone experiences these chills with equal intensity, and people who do tend to score higher on measures of openness to experience.
Low Blood Sugar
When blood glucose drops below about 70 mg/dL, your body treats it as an emergency. Your nervous system kicks into a stress response that can produce shaking, sweating, a racing heartbeat, anxiety, and a chill-like sensation. The shakiness people feel during a blood sugar crash isn’t exactly the same as cold-triggered shivering, but it’s easy to confuse the two because the muscle tremors feel similar.
This is most common in people with diabetes who take insulin or certain medications, but it can happen to anyone who goes too long without eating, exercises intensely on an empty stomach, or drinks alcohol without food. If you notice chills paired with sudden hunger, irritability, and dizziness, low blood sugar is worth considering.
Thyroid Problems
Your thyroid gland produces hormones that are major regulators of metabolism and heat production. When thyroid hormone levels drop too low, a condition called hypothyroidism, your basal metabolic rate slows down. You simply produce less internal heat throughout the day.
The result is a persistent feeling of being cold that doesn’t match your environment. People with hypothyroidism often describe needing extra layers when everyone around them is comfortable, or feeling chills in rooms that are objectively warm. Unlike fever chills, which come in waves and resolve, thyroid-related cold intolerance tends to be constant and gradual in onset. It’s often accompanied by fatigue, weight gain, dry skin, and sluggish digestion.
Iron Deficiency and Anemia
Iron-deficiency anemia reduces the number of functional red blood cells carrying oxygen from your lungs to your tissues. This has a direct effect on your ability to stay warm. Without adequate oxygen delivery, your body can’t maintain its normal metabolic rate, which is the chemical engine that generates body heat. It also impairs your ability to constrict blood vessels near the skin surface, a key heat-conserving mechanism.
The combination means you lose body heat faster and produce less of it. Studies on subjects with iron-deficiency anemia show measurable drops in body temperature during cold exposure compared to people with normal iron levels. When red blood cell counts are restored, thermoregulatory function returns to normal. If you’re frequently cold and also dealing with fatigue, pale skin, or shortness of breath during mild activity, anemia could be the underlying issue.
Other Common Triggers
Several other situations can produce chills that don’t fit neatly into the categories above. Intense exercise can temporarily disrupt your thermoregulation, especially if you stop suddenly and sweat evaporates rapidly from your skin. Panic attacks and acute stress activate the sympathetic nervous system in ways that can cause shivering, goosebumps, and a sensation of cold. Certain medications, particularly during withdrawal from opioids or alcohol, can cause dramatic chills as the nervous system rebounds.
Urinary tract infections are a surprisingly common cause of chills, especially in older adults, sometimes appearing before any urinary symptoms. And some people simply have a lower tolerance for cold due to differences in body composition, since people with less muscle mass and body fat generate and retain less heat.