Cold chills are your body’s rapid-fire attempt to generate heat, driven by involuntary muscle contractions and blood-vessel constriction that your brain orchestrates from a thermoregulatory control center called the preoptic area. The most common trigger is infection: your immune system deliberately raises the body’s temperature set point, and the resulting gap between where your temperature is and where your brain now wants it to be is what makes you shiver and feel cold even in a warm room. But infections are far from the only cause, and the intensity of your chills turns out to be a surprisingly useful clue about how serious the underlying problem is.
How the Brain Creates Chills
Your brain constantly monitors core body temperature and compares it to an internal set point, much like a thermostat. When actual temperature drops below that set point, the brain triggers a cascade of heat-generating responses. The most obvious is shivering, which is essentially rapid, involuntary muscle contraction. The muscles burn fuel and produce heat as a byproduct, primarily through the same molecular machinery used in voluntary movement.1Diabetes & Metabolism Journal. Obesity and Metabolic Syndrome Skeletal Muscle Thermogenesis and Its Role in Whole Body Energy Metabolism At the same time, blood vessels near the skin’s surface constrict to keep warm blood closer to your core. Together, these responses create the classic chill experience: you feel cold on the outside even while your internal temperature is stable or rising.
During an infection, this system gets hijacked in a specific way. Immune cells release signaling molecules called pyrogenic cytokines, particularly interleukin-1, interleukin-6, and tumor necrosis factor-alpha.2PubMed. Cytokines and fever. Mechanisms and sites of action These molecules ultimately cause the brain’s vascular cells to produce prostaglandin E₂, which then acts on the preoptic area and effectively turns the thermostat up.3Journal of Endotoxin Research. Review: Infection, fever, and exogenous and endogenous pyrogens: some concepts have changed Your normal body temperature of around 37°C suddenly registers as “too cold” by the brain’s new standard, so it fires off all the same warming mechanisms it would use if you had fallen into a lake. You shiver, your skin pales, and you pile on blankets even though a thermometer might already show a fever. The chills subside once your temperature catches up to the new, elevated set point. Research has shown this process involves at least two brain regions working together: one driving the automatic shivering and blood-vessel responses, and another driving the behavioral urge to seek warmth, curl up, and cover yourself.4Medical Xpress. Fever chills explained: How brain signals push warmth-seeking to fight infection
Why Fever Exists at All
Fever is not a malfunction. It is an ancient defense strategy conserved across warm-blooded and cold-blooded vertebrates for over 600 million years, and studies consistently show it confers a survival benefit during infection.5PubMed Central. Fever and the thermal regulation of immunity: the immune system feels the heat Higher body temperatures speed up immune-cell activity, impair the replication of many pathogens, and enhance certain antimicrobial processes. The chills you feel are the cost of rapidly ramping temperature up. This is worth keeping in mind: a moderate fever with mild chills from a common viral illness is your immune system doing its job, not an emergency. The question is always what is causing the fever and how intense the response has become.
When Chills Point to a Serious Infection
Not all chills are created equal. Clinicians have long distinguished between mild chilliness, moderate chills where you need a heavy blanket, and shaking chills (sometimes called rigors) where your whole body shakes uncontrollably even under thick covers.6PubMed. The degree of chills for risk of bacteremia in acute febrile illness That distinction matters a great deal. Rigors are one of the strongest bedside predictors that bacteria have entered the bloodstream. In one emergency department study, the presence of rigors was independently associated with about a fourteen-fold increase in the odds of bacteremia among febrile adults, while even lesser chills carried roughly six-fold increased odds.7PubMed. Prediction of community-onset bacteremia among febrile adults visiting an emergency department: rigor matters
A systematic review looking at shaking chills as a diagnostic marker for bacteremia found that while sensitivity was modest (about 37%), specificity was high (about 87%), meaning most people without bacteremia do not experience true rigors.8PubMed Central. Utility of shaking chills as a diagnostic sign for bacteremia in adults: a systematic review and meta-analysis In practical terms, a person can have bacteremia without shaking chills, but if shaking chills are present, clinicians take that seriously as a red flag.
Malaria provides a vivid example of infection-driven chills. The malaria parasite multiplies inside red blood cells and then ruptures them in synchronized waves, every 48 or 72 hours depending on the species. Each rupture releases parasite toxins that trigger a massive surge of pyrogenic cytokines.9IntechOpen. Pathophysiological Mechanisms of Malaria and Their Clinical Consequences in Humans The result is the classic malarial paroxysm: a cold stage with violent chills and shivering, followed by a hot stage of high fever and flushing, then a sweating stage where the fever breaks. Research tracking tumor necrosis factor levels during these episodes found that TNF spikes preceded each temperature rise by about 30 to 60 minutes, strongly suggesting the cytokine surge directly drives the chill-fever cycle.10PubMed. Dynamics of fever and serum levels of tumor necrosis factor are closely associated during clinical paroxysms in Plasmodium vivax malaria
Non-Infectious Causes You Might Not Expect
Infection accounts for most chills that come with fever, but several other conditions can produce the sensation of being cold or shivery without any pathogen involved.
Thyroid Problems
An underactive thyroid (hypothyroidism) slows metabolism and impairs your body’s ability to generate heat when you need it. People with hypothyroidism commonly report feeling cold all the time, even in environments others find comfortable. When thyroid function is restored to normal, the body’s capacity to produce heat in response to cold exposure roughly doubles.11PubMed Central. Resolution of Hypothyroidism Restores Cold-Induced Thermogenesis in Humans If you have persistent cold sensitivity with fatigue, dry skin, and unexplained weight gain, thyroid testing is worth discussing with your doctor.
Low Blood Sugar
Hypoglycemia can provoke shivering and a clammy, cold feeling. Research on thermoregulatory responses during low blood sugar found that hypoglycemia significantly shifts the core temperature threshold at which shivering begins, meaning the body starts shivering at a much higher temperature than it normally would.12PubMed Central. Effect of hypoglycemia on thermoregulatory responses In other words, low blood sugar makes your body behave as though it is colder than it actually is. This is especially relevant for people with diabetes who use insulin, since hypoglycemic episodes are a recurring risk. The chills typically resolve once blood sugar is brought back up.
Menopause and Hormonal Shifts
Hot flashes get most of the attention during menopause, but chills often accompany them or occur independently. Fluctuating estrogen levels disrupt the brain’s temperature-control circuitry, narrowing the zone of temperatures the body considers “normal” and triggering exaggerated heat-loss or heat-gain responses at the slightest provocation.13PubMed. Understanding the pathophysiology of vasomotor symptoms (hot flushes and night sweats) that occur in perimenopause, menopause, and postmenopause life stages A hot flash is the body suddenly deciding it is too warm and flooding the skin with blood to dump heat; the chill that follows is the overcorrection, where the sudden heat loss leaves you cold and shivery. Beyond estrogen, research points to involvement of several brain signaling systems, including neurokinin B and serotonin, in generating these symptoms.14PubMed Central. Menopausal Hot Flashes: A Concise Review
Anxiety and Panic Attacks
Chills and hot flashes are recognized features of panic attacks. In factor analyses of panic attack symptoms, chills load alongside sweating, trembling, nausea, and dizziness as part of a somatic subtype.15PubMed. Panic attack symptom dimensions and their relationship to illness characteristics in panic disorder Acute anxiety activates the sympathetic nervous system, which constricts blood vessels in the skin (diverting blood to muscles for a “fight or flight” response), producing a genuine sensation of cold. These chills tend to be brief and stop when the panic subsides, but they can feel alarming in the moment. If you experience unexplained chills alongside racing heart, shortness of breath, and dread, anxiety is a realistic explanation.
Chills After Surgery or Blood Transfusions
Postoperative shivering is remarkably common. General anesthetics impair the brain’s thermoregulatory control in a dose-dependent manner, allowing core temperature to drop as heat redistributes from the body’s core to the cooler periphery.16PubMed Central. Temperature monitoring and perioperative thermoregulation When the anesthetic wears off and the brain’s thermostat “wakes up,” it finds the body colder than expected and launches aggressive shivering. Interestingly, shivering sometimes occurs even when patients are not hypothermic, suggesting that pain and the withdrawal of short-acting opioids may also contribute.17PubMed Central. Postanaesthetic shivering – from pathophysiology to prevention Warming blankets and, when needed, small doses of specific medications are the standard approach to managing it.
Blood transfusions are another medical setting where chills arise. Febrile nonhemolytic transfusion reactions produce a temperature rise of 1°C or more during or shortly after a transfusion. These reactions are thought to result from cytokines that accumulate in the blood product during storage, or from the recipient’s immune system reacting to donor white blood cell components.18PubMed Central. What every physician should know about transfusion reactions Although usually benign, chills during a transfusion always warrant stopping the infusion and evaluating, because more dangerous hemolytic reactions can present the same way initially.
When Chills Happen Without Clear Illness
Sometimes people experience a wave of chills without any obvious trigger. A few normal scenarios can explain this. Your core temperature naturally dips in the evening as melatonin rises and your body prepares for sleep. Research has found that light exposure patterns during the day influence how sharply this evening temperature drop occurs and how sensitive you are to cold during that window.19Journal of Physiological Anthropology and Applied Human Science. How Does Light Intensity Influence Evening Dressing Behavior in the Cold? Bright light during the day can amplify the evening temperature dip via higher melatonin elevation, potentially making you feel chillier at night than you otherwise would.
Fatigue, dehydration, and skipping meals can also lower your functional body temperature enough to cause brief chills. These are typically mild and resolve with warmth, food, or rest. The chills worth paying attention to are the ones that persist, escalate, or come with other symptoms like a rapid heart rate, confusion, or pain.
How Chills Differ in Children and Older Adults
The chill-and-fever response does not look the same at every age. Young children, especially those between six months and five years, can sometimes develop febrile seizures during the rapid temperature rise that accompanies a fever with chills. These seizures peak around 18 months of age, and a family history of febrile seizures increases the risk substantially: about 20% in siblings and 33% when a parent has the history.20PubMed Central. An overview of risk factors in children with febrile seizures A simple febrile seizure typically involves stiffening and rhythmic jerking that lasts under five minutes, followed by brief drowsiness.21PubMed Central. Febrile seizures: an overview While terrifying to witness, simple febrile seizures are generally benign and do not cause brain damage. A seizure lasting longer than 15 minutes, involving only one side of the body, or recurring within 24 hours is classified as complex and warrants urgent medical evaluation.
At the other end of the age spectrum, elderly people present a different challenge. Impaired shivering and vasomotor responses mean older adults may not mount the same visible chill response even when they are seriously ill.22PubMed Central. Shaking chills and high body temperature predict bacteremia especially among elderly patients People with cognitive impairment may be unable to report that they feel cold. This matters because the absence of fever and chills in an elderly person does not rule out serious infection the way it might in a younger adult. Caregivers and clinicians need to rely on other clues, such as confusion, low blood pressure, or a rapid breathing rate, rather than waiting for the classic fever-and-chills picture.
What People Actually Experience During Fever Chills
Descriptions of chills tend to sound uniform in medical textbooks, but patient interviews reveal a more layered experience. In one qualitative study, about 79% of people hospitalized with fever reported feeling cold, having chills, shivering, or shaking. Several participants described the chills as a predictive signal: they could feel them coming before the fever itself registered, interpreting the onset of cold sensations as a warning that their temperature was about to climb.23PubMed Central. Beyond Intuition: Patient Fever Symptom Experience Others associated chills with reaching a particular fever intensity, noticing that mild fevers did not produce them but higher ones did. This subjective experience aligns with the physiology: chills are most pronounced during the rapid-rise phase of a fever, when the gap between actual body temperature and the brain’s new set point is widest.
This has a practical takeaway. If you notice sudden, intense chills, your body may be mounting a strong febrile response. It is worth checking your temperature, but also paying attention to the trajectory. Chills that come and go with a garden-variety cold are expected. Chills that are severe and shaking, come on suddenly in a previously well person, or persist alongside confusion, rapid breathing, or a very high temperature deserve prompt medical attention.
Skeletal Muscle and Heat Production Beyond Shivering
Shivering is the obvious heat-generating mechanism, but your muscles have a subtler trick as well. Research has identified a process in skeletal muscle where a protein called sarcolipin uncouples a calcium pump in muscle cells, causing the pump to cycle without doing useful work and releasing heat as a byproduct.24PubMed Central. The role of skeletal-muscle-based thermogenic mechanisms in vertebrate endothermy This is a form of non-shivering thermogenesis: heat production that does not require visible muscle contracting. It helps explain why you can sometimes feel warmer after a sustained cold exposure even before visible shivering kicks in, and why people with more lean muscle mass tend to tolerate cold environments somewhat better. Brown fat uses a similar uncoupling principle, and both systems work alongside shivering to defend body temperature.
The practical implication is that factors reducing muscle mass, like prolonged bed rest, aging, or severe illness, also reduce the body’s capacity to generate heat through both shivering and non-shivering mechanisms. This is one reason frail or severely ill patients are particularly vulnerable to hypothermia and may not display the dramatic chill response that would otherwise signal a problem.
Red Flags That Warrant Medical Attention
Most episodes of chills are benign and tied to viral illnesses that resolve on their own. But certain patterns should prompt you to seek care sooner rather than later:
- Shaking rigors: Whole-body shaking you cannot control, especially if it comes on suddenly, is associated with a much higher likelihood of bacteria in the bloodstream compared to milder chilliness.
- Fever above 39.4°C (103°F) in adults: High fevers combined with chills increase the probability of bacteremia, particularly in people over 65.
- Altered mental status: Confusion, unusual drowsiness, or difficulty speaking alongside chills and fever can indicate sepsis, which requires emergency treatment.
- Recent surgery or medical procedure: Chills developing within days of a surgery, catheter placement, or any invasive procedure can signal a healthcare-associated infection.
- Immunocompromise: People undergoing chemotherapy, taking immunosuppressant drugs, or living with HIV should treat any new chills with fever as urgent, because their bodies may not mount the usual warning signs of severe infection.
- Chills with localized pain: Shaking chills accompanied by flank pain (possible kidney infection), abdominal pain (possible abdominal abscess or cholangitis), or severe headache with neck stiffness (possible meningitis) all warrant rapid evaluation.
For children, a febrile seizure during a chill-and-fever episode should be evaluated by a pediatrician, especially the first time it happens, even though most simple febrile seizures do not cause lasting harm. Any seizure lasting longer than five minutes, involving only one side of the body, or occurring more than once in 24 hours needs emergency attention.21PubMed Central. Febrile seizures: an overview
Chills Without Fever and What They Suggest
Chills in the absence of any measurable fever point toward a different set of causes. The hormonal shifts of menopause and perimenopause, as discussed earlier, are among the most common. Hypothyroidism, anemia, and low blood sugar can each produce cold sensations and even visible shivering without an elevated temperature. Anxiety-driven chills likewise occur without fever. And simple environmental causes should not be overlooked: a drafty room, wet clothing, or a sudden temperature drop can trigger shivering in a healthy person with no underlying illness at all.
Persistent, unexplained chills without fever that last days or weeks and do not respond to warming up are worth investigating. In rare cases they can be an early symptom of certain blood cancers, particularly lymphoma, where the immune system releases cytokines that cause fever-like symptoms (often called “B symptoms”) even when a thermometer reads normal or nearly normal. This is an uncommon scenario, but it is the reason doctors pay attention when someone reports recurrent chills alongside unexplained weight loss or drenching night sweats.