Chills happen when your body either loses heat faster than it can produce it or when your brain’s internal thermostat gets tripped by infection, hormones, stress, or other triggers. The sensation ranges from a mild cold feeling to full-body shaking, and the causes span from completely harmless to medically urgent. Understanding what’s behind your chills depends on context: whether you also have a fever, how long they last, and what else is going on in your body.
How Your Body Produces Chills
Your brain maintains a tight core body temperature, typically around 98.6°F. When something disrupts that set point or when your actual body temperature drops below it, your brain triggers a cascade of responses. Blood vessels near your skin constrict to keep warm blood closer to your vital organs. Your muscles begin contracting rapidly and involuntarily, which is shivering. The cold, goosebump-covered feeling you get alongside this is what most people call “chills.”
This system can be activated by actual cold exposure, but it also fires in situations where your body temperature is technically normal. That’s why you can get chills sitting in a warm room. The trigger isn’t always temperature itself; it’s your brain’s perception that something is off.
Fever and Infection
The most common reason for sudden, noticeable chills is your immune system fighting an infection. When your body detects bacteria or viruses, it releases chemicals that temporarily raise the thermostat in your brain. Your new “target” temperature might be 101°F or 102°F, but your actual body temperature is still 98.6°F. Your brain reads that gap as being too cold, so it triggers shivering to generate heat and get you up to the new set point. This is why chills often come right before a fever breaks.
Most viral infections like the flu, COVID, or a common cold cause mild to moderate chills that resolve as the illness passes. But the intensity of chills matters. Researchers classify them on a spectrum: mild chills feel like needing an extra jacket, moderate chills make you reach for a thick blanket, and shaking chills (called rigors) involve whole-body trembling even under heavy covers. Rigors with a fever above 100.4°F can signal a serious bacterial infection in the bloodstream and warrant urgent medical attention.
Anxiety and the Fight-or-Flight Response
Stress and anxiety are surprisingly common causes of chills, even without any illness. During a panic attack or a moment of intense anxiety, your nervous system activates the fight-or-flight response. Blood rushes toward your heart, lungs, and core organs, leaving your extremities with reduced blood flow. Your fingers, toes, and skin surface get cold, and you may feel tingling alongside the chill.
Many people report chills or cold flashes during panic attacks. The exact mechanism isn’t fully understood, but the leading theory ties it to the same blood redistribution that causes numbness and tingling in your hands during high anxiety. If you notice chills during stressful moments, presentations, or episodes of worry, your nervous system is likely the culprit rather than an infection.
Low Thyroid Function
If you feel cold constantly, not just during illness, your thyroid may be underperforming. The thyroid gland controls your metabolic rate, which is essentially how much heat your body generates at rest. Hypothyroidism slows metabolism and drops your core body temperature, leaving you with a low tolerance for cold that other people around you don’t seem to share. Chills from thyroid problems tend to be persistent and low-grade rather than sudden and dramatic. They often come with fatigue, weight gain, dry skin, and sluggishness.
Iron Deficiency and Anemia
Iron plays a larger role in body temperature than most people realize. When you’re iron-deficient or anemic, your blood carries less oxygen from your lungs to your tissues. This directly impairs two key heat-preserving processes: your blood vessels lose the ability to constrict properly near the skin surface, and your metabolic rate drops because cells can’t burn fuel efficiently without adequate oxygen.
Iron deficiency also appears to interfere with the thyroid’s response to cold, compounding the problem. On top of that, low iron may alter levels of brain chemicals involved in temperature regulation, including dopamine, serotonin, and norepinephrine. The result is a body that struggles to both generate and conserve heat. If you’re frequently cold and also dealing with fatigue, pale skin, or shortness of breath, iron levels are worth checking.
Low Blood Sugar
When your blood sugar drops too low, your body releases adrenaline and related stress hormones to push glucose levels back up. This hormonal surge produces trembling, sweating, a rapid heartbeat, and anxiety, a combination that can feel a lot like chills. The shaking from low blood sugar tends to come on quickly and is often accompanied by sudden hunger, lightheadedness, or irritability. Eating something with carbohydrates usually resolves the sensation within 15 to 20 minutes.
Hormonal Changes During Menopause
Hot flashes get most of the attention during menopause, but cold flashes are common too. Some women experience more cold flashes than hot ones. These episodes happen because fluctuating estrogen levels destabilize the brain’s temperature control center, causing it to misread normal body temperature as too hot or too cold. A cold flash can follow immediately after a hot flash, as the body overcorrects after a burst of sweating, or it can occur independently. The unpredictability of these episodes is one of the more frustrating aspects of perimenopause and menopause.
Dehydration
Your body relies on adequate fluid volume to regulate temperature. When you’re dehydrated, your blood volume drops, which makes it harder for your cardiovascular system to distribute heat evenly. Cleveland Clinic lists chills as a recognized symptom of dehydration, alongside headache, dark urine, and dizziness. This is especially relevant during exercise, hot weather, or illness with vomiting and diarrhea, all situations where fluid loss accelerates. Rehydrating gradually with water and electrolytes typically resolves dehydration-related chills.
Post-Surgery Shivering
If you’ve recently had surgery and are wondering why you’re shaking uncontrollably, you’re not alone. Postoperative shivering affects anywhere from 5% to 65% of surgical patients depending on the type of procedure, the anesthesia used, and how long the operation lasted. Cold operating rooms (below about 70°F), longer surgeries, large volumes of room-temperature IV fluids, and older age all increase the risk. Anesthesia suppresses your body’s normal temperature regulation, so your core temperature often drifts downward during the procedure. The shivering that follows is your body’s attempt to reheat itself. It’s uncomfortable but usually resolves within 30 to 60 minutes with warming blankets.
Sleep Deprivation and Fatigue
Lack of sleep disrupts your body’s ability to maintain a stable temperature. Your core temperature naturally dips at night as part of your circadian rhythm, and chronic sleep deprivation can make your thermoregulation less precise overall. If you’ve noticed you feel colder and more chill-prone during periods of poor sleep, the connection is real. Exhaustion also lowers immune function, which can make you more susceptible to the mild infections that cause feverish chills.
When Chills Signal Something Serious
Most chills are harmless and self-limiting. But certain patterns deserve prompt attention. Shaking chills with a high fever, especially if you have a weakened immune system, a recent surgery, or a urinary catheter, can indicate bacteria in the bloodstream. Chills combined with confusion, rapid breathing, or a heart rate above 100 beats per minute suggest your body is under significant stress. Persistent chills without an obvious cause that last more than a few days, particularly alongside unexplained weight loss or night sweats, are worth investigating with blood work to check for thyroid problems, anemia, infections, or other underlying conditions.