Chills are your body’s rapid-fire method of generating heat, typically through involuntary muscle contractions that range from a slight shiver to teeth-chattering shakes. The causes span a wide spectrum, from the familiar onset of a fever to hormonal shifts, low blood sugar, emotional responses to music, and even the aftermath of surgery. Most episodes are harmless and short-lived, but certain patterns of chills, especially shaking rigors that recur or accompany other warning signs, deserve prompt medical attention.
How Chills Actually Work
When your brain’s thermostat detects that your core temperature is too low, or that it needs to be higher (as in a fever), it triggers shivering. This is not random trembling. Your skeletal muscles contract and relax in rapid bursts, converting chemical energy into heat. Research using skin-temperature clamping in healthy adults shows that shivering-based heat production rises in proportion to how much skin temperature drops, and that the body recruits progressively more muscle groups as the cold stimulus increases.1Cell Metabolism. Shivering, but not adipose tissue thermogenesis, increases as a function of mean skin temperature in cold-exposed men and women Women tend to recruit more lower-body muscles and activate a greater number of motor units during shivering, though the overall thermoregulatory response is similar between sexes.
Your body also has a non-shivering way of making heat through brown adipose tissue, sometimes called brown fat. But unlike shivering, which scales up smoothly with cold exposure, brown fat activation tends to work more like an on-off switch rather than a dial. The balance between these two systems varies widely from person to person, which partly explains why some people seem to get the chills far more easily than others.2PubMed Central. Shivering thermogenesis in humans: Origin, contribution and metabolic requirement
Fever and Infection
The most common reason people notice chills is the onset of a fever. When your immune system detects an infection, it releases signaling molecules called pyrogens that effectively raise the thermostat in your brain. Your body suddenly “thinks” its current temperature is too low, so it kicks off the same shivering response you would get standing outside in the cold. You feel freezing even though your actual temperature is climbing. This is why you might pile on blankets and shiver violently at the start of an illness, only to feel burning hot an hour later once your temperature catches up to the new set point.
A single episode of chills at the start of an acute infection is common and not necessarily alarming on its own. It can happen with any sudden-onset fever, though urinary tract infections are a particularly frequent trigger. The picture changes, however, when chills repeat in waves or escalate into violent, full-body shaking known as rigors. Repeated rigors tend to show up in a more limited set of serious infections, many of which involve the bloodstream, the biliary system, or deep-seated abscesses.
When Shaking Chills Point to Bacteremia
Bacteremia, the presence of bacteria in the bloodstream, is one of the more dangerous conditions associated with chills. Shaking chills specifically (not just feeling cold, but the involuntary, teeth-chattering, bed-shaking kind) are a red flag. A systematic review and meta-analysis found that shaking chills are a highly specific predictor of bacteremia, meaning that when someone has true rigors, there is a meaningfully elevated chance that bacteria are circulating in their blood.3PubMed Central. Utility of shaking chills as a diagnostic sign for bacteremia in adults: a systematic review and meta-analysis The catch is that the sensitivity is lower: plenty of people with bacteremia never experience shaking chills, so the absence of rigors does not rule out a bloodstream infection.
A prospective study of emergency department patients with abnormal vital signs quantified this further, finding that patients who reported shaking chills had roughly three to four times the odds of having a positive blood culture compared to those without chills, even after adjusting for age and sex.4PubMed Central. Risk of bacteremia in patients presenting with shaking chills and vomiting – a prospective cohort study This is why emergency physicians take true rigors seriously: the finding often prompts blood cultures and early antibiotic treatment before lab results are back.
The practical takeaway here is about the quality of the chills, not just the fact that you feel cold. A mild shiver with a cold is unremarkable. Violent, uncontrollable shaking, especially if it comes in multiple episodes, especially if accompanied by high fever, rapid heartbeat, or confusion, is the pattern that warrants urgent evaluation.
Thyroid Problems and Cold Sensitivity
If you find yourself reaching for a sweater when everyone else is comfortable, your thyroid may be part of the story. The thyroid gland plays a central role in setting your metabolic rate and, by extension, your ability to generate heat. People with an underactive thyroid (hypothyroidism) often report feeling persistently cold and may shiver more readily in mildly cool environments.
This connection has measurable physiological backing. A study of patients with hypothyroidism found that once thyroid hormone levels were restored to normal, their cold-induced thermogenesis roughly doubled, with median heat production jumping from about 55 to about 111 kilocalories per day during cold exposure.5PubMed Central. Resolution of Hypothyroidism Restores Cold-Induced Thermogenesis in Humans In other words, a sluggish thyroid meaningfully impairs your body’s ability to warm itself, and treating the underlying condition restores it. If you are consistently more cold-sensitive than the people around you, it is worth having your thyroid function checked.
Anemia and Iron Deficiency
Anemia, particularly iron-deficiency anemia, is another underappreciated reason people feel chilly. The link is straightforward: red blood cells carry oxygen to tissues that burn fuel to produce heat. When your red blood cell count is low, less oxygen gets delivered, and heat production suffers. Animal research has demonstrated that restoring hematocrit (the proportion of blood made up of red blood cells) to adequate levels reliably restores normal thermoregulatory performance, while transfusing to lower hematocrits induces cold sensitivity even in otherwise healthy subjects.6PubMed. Effect of iron-deficiency anemia on hormone levels and thermoregulation during cold exposure
There is also a hormonal layer to this: anemia appears to compromise the thyroid hormone response to cold, creating a kind of double hit where both the oxygen delivery and the hormonal signaling needed to ramp up heat production are blunted. For someone who is frequently cold and also experiences fatigue, pale skin, or shortness of breath, iron levels and a complete blood count are reasonable things to discuss with a doctor.
Low Blood Sugar
A sudden drop in blood sugar can trigger chills that feel a lot like being cold, though the mechanism is different. During hypoglycemia, the body releases stress hormones including adrenaline, which dilates blood vessels near the skin and triggers sweating. Research in human subjects found that when blood sugar was lowered via insulin infusion, heat production actually increased (from about 5.1 to about 6.3 kilojoules per minute), but core temperature still fell by about half a degree Celsius because peripheral vasodilation and sweating caused net heat loss.7Portland Press (Clinical Science). Hypoglycaemia, Hypothermia and Shivering in Man The result is a paradox: your body is burning more energy, yet you feel cold and may start to shiver because your skin and core are actually cooling down.
This matters most for people who take insulin or certain diabetes medications, but it can also happen in anyone who goes a long time without eating, especially during intense physical activity. The chills usually come alongside other hypoglycemic symptoms like shakiness, sweating, irritability, and difficulty concentrating. Eating something with carbohydrates typically resolves both the blood sugar drop and the chills within minutes.
Menopausal Hot Flashes and the Chill That Follows
Menopause-related chills are a classic example of the body overcorrecting. A hot flash is a sudden, exaggerated heat-loss response: blood vessels in the skin dilate, you flush, you sweat. But once that wave passes, the rapid heat loss can leave you feeling genuinely cold, sometimes with visible goosebumps and shivering. The chill is the aftermath of the flash, not a separate event.
Hot flashes, defined as transient sensations of heat, sweating, flushing, and chills lasting roughly one to five minutes, are among the most common symptoms of menopause.8PubMed Central. Menopausal Hot Flashes: A Concise Review The underlying problem is a disruption in the brain’s temperature-control circuit, driven primarily by falling estrogen levels. Changing estrogen impacts multiple components of temperature homeostasis, effectively narrowing the zone of core temperatures the body tolerates without triggering a corrective response.9PubMed. Understanding the pathophysiology of vasomotor symptoms (hot flushes and night sweats) that occur in perimenopause, menopause, and postmenopause life stages A tiny fluctuation in core temperature that your body would previously have ignored now sets off an alarm, producing a hot flash, followed by the rebound chill.
Interestingly, estrogen levels alone do not fully explain who gets these symptoms and who does not. Blood estrogen levels are similar in symptomatic and asymptomatic menopausal women, suggesting that the brain’s sensitivity to hormonal changes, rather than the absolute hormone level, determines who suffers.10PubMed. Physiology of hot flashes
Chills After Surgery or Anesthesia
Post-anesthesia shivering is one of the most common complaints after surgery, and it is often worse than patients expect. The primary cause is hypothermia that develops during the procedure itself: general anesthesia suppresses the body’s normal thermoregulatory responses, allowing core temperature to drift downward while the patient lies motionless in a cool operating room.11PubMed. Postanaesthetic shivering: epidemiology, pathophysiology, and approaches to prevention and management Once the anesthesia wears off and the brain’s thermostat comes back online, it registers the temperature deficit and triggers intense shivering to close the gap.
This is more than just uncomfortable. Shivering dramatically increases oxygen consumption and cardiac workload, which can be risky for people with heart or lung conditions. That is why modern surgical practice emphasizes active warming during procedures, using forced-air warming blankets and warmed intravenous fluids. If you have surgery coming up, it is reasonable to ask your surgical team about their warming protocols.
Chills During and After Exercise
Getting the chills during or immediately after exercise seems counterintuitive since your body is generating a lot of heat, but it happens in several scenarios. One is exercising in cold or wet conditions: once you stop moving and your metabolic heat production drops, the combination of sweat-soaked clothing and wind chill can pull heat away from your body faster than you replace it. Shivering kicks in within minutes.
A less obvious scenario involves exertional heat illness. Chills are listed among the common signs and symptoms of exertional heat illness by the American College of Sports Medicine, and they often resolve with rapid cooling.12Current Sports Medicine Reports. ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity This is another example of the body’s thermostat misfiring: during heat illness, the brain may send conflicting signals, triggering chills even as core temperature is dangerously elevated. Anyone who develops chills during heavy exertion in hot weather, especially alongside nausea, confusion, or cessation of sweating, should stop exercising, get into shade, and cool down immediately.
Emotional and Aesthetic Chills
Not all chills are about temperature. Many people experience shivers, goosebumps, or a tingling sensation down the spine in response to music, art, awe-inspiring scenery, or emotionally charged moments. These “aesthetic chills” or “frissons” are a well-studied phenomenon in psychology and neuroscience. They involve both a subjective emotional experience and a measurable physical response, including piloerection (goosebumps) and sometimes actual shivering.13PubMed Central. Thrills, chills, frissons, and skin orgasms: toward an integrative model of transcendent psychophysiological experiences in music
Neuroimaging research has begun to map what happens in the brain during these moments, finding that aesthetic chills appear to involve reward circuits similar to those activated by food or social bonding. They offer a unique window into how bodily sensations shape conscious emotional experience, because the physical chill and the emotional feeling arise together in a way that is hard to separate.14PubMed Central. The neurobiology of aesthetic chills: How bodily sensations shape emotional experiences These chills are entirely benign. If a piece of music gives you goosebumps, that is your brain’s reward system lighting up, not a sign of illness.
Some people report emotional chills more frequently than others. Personality traits like openness to experience seem to play a role, as does the specific structure of the music or stimulus (unexpected harmonic shifts, crescendos, and moments of resolution are particularly effective triggers). If you rarely get music chills, there is nothing wrong with you; if you get them constantly, you are probably just wired to have a stronger emotional-physical coupling in response to aesthetic stimuli.
Your Body Clock and Nighttime Chills
Your core body temperature is not constant throughout the day. It follows a circadian rhythm, rising during the afternoon, peaking in the early evening, and then falling steadily overnight to reach its lowest point around the time you wake up.15PubMed. The circadian rhythm of human body temperature – Clinical implications and review of the literature This natural temperature dip overnight is one reason you might feel chilly in the early morning hours even in a warm room, or why you wake up reaching for a blanket you kicked off at midnight.
For people who are already on the edge of feeling cold because of anemia, low thyroid function, or low body weight, this circadian dip can push them into noticeable shivering. Night sweats followed by chills are also a pattern in certain infections (particularly tuberculosis and some lymphomas), so persistent, drenching night sweats that leave you shivering afterward warrant a medical conversation even if each individual episode resolves quickly.
Drug-Related and Withdrawal Chills
A number of medications can cause chills as a side effect, particularly drugs administered intravenously such as certain antibiotics, chemotherapy agents, and blood products. The mechanism varies: some drugs directly trigger an immune-like response, while others cause rapid temperature shifts that the body interprets as a chill-worthy event.
Substance withdrawal is another potent trigger. Withdrawal from opioids, alcohol, and certain sedatives commonly produces intense chills and shivering as part of a broader autonomic storm that includes sweating, rapid heart rate, and anxiety. The phrase “going cold turkey” literally references the goosebumps and cold, clammy skin associated with opioid withdrawal. These chills are a sign that the nervous system is in overdrive after losing a substance it had adapted to, and they typically peak within the first few days of withdrawal before gradually improving.
When Chills Actually Warrant Medical Attention
Most chills resolve on their own once the trigger passes, whether that is warming up, recovering from a mild infection, eating something, or waiting for a hot flash to subside. But some patterns should prompt you to seek care sooner rather than later:
- True rigors: Uncontrollable, violent shaking that you cannot stop by bundling up. This is distinct from ordinary shivering and is associated with a meaningfully higher likelihood of bacteremia.
- Recurrent episodes: Chills that come in repeated waves over hours or days, especially with fever, suggest a systemic infection that may involve the bloodstream, heart valves, or an abscess.
- High fever with confusion: Chills paired with a temperature above 103°F (39.4°C) and any change in mental status such as disorientation, excessive drowsiness, or agitation warrant emergency evaluation.
- Post-surgical chills: Expected to some degree, but prolonged or severe shivering after a procedure should be reported to your care team since it increases cardiac strain.
- Chills during heat exposure: Shivering while exercising in hot weather is a paradoxical red flag for heat illness and calls for immediate cooling.
- Persistent cold sensitivity: Feeling chronically colder than the people around you, especially with fatigue, weight changes, or heavy menstrual periods, is worth investigating for thyroid dysfunction or anemia.
Context matters enormously. A single chill at the onset of a cold during flu season is one of the most mundane events in medicine. The same chill in someone who recently had surgery, has a compromised immune system, or just returned from a region where malaria is endemic is a very different story.
Why Some People Seem to Get Chills More Easily
Individual variation in chill susceptibility is real and comes from multiple overlapping factors. Body composition plays a role: people with less subcutaneous fat lose heat faster through the skin. Muscle mass matters too, since muscles are the engines of shivering, and people with more muscle can generate heat more efficiently. Hormonal differences, iron stores, thyroid function, and even how much brown fat a person carries all shift the threshold at which shivering kicks in.2PubMed Central. Shivering thermogenesis in humans: Origin, contribution and metabolic requirement
Age also shifts the equation. Older adults tend to have a reduced shivering response and less effective vasoconstriction, which means they lose heat faster but may not generate compensatory chills as readily. Paradoxically, this can make them more vulnerable to hypothermia in situations where younger people would simply feel uncomfortable and shiver their way back to warmth. Very young children face a similar imbalance from the other direction: they have a high surface-area-to-mass ratio and limited muscle mass, so they chill quickly despite having relatively active brown fat.
If you are someone who seems to get chills at the slightest provocation, there may be no single explanation. It is often a combination of body type, metabolic rate, hormonal milieu, and habitual cold exposure. Chronic cold exposure does gradually shift the balance toward more brown fat activation and less shivering, which is one reason people who work outdoors in winter often report adapting to the cold over time. But “toughing it out” is not medical advice, and persistent unexplained cold sensitivity still deserves a workup to rule out correctable causes like thyroid or iron problems.