Shivering is your body’s emergency furnace: involuntary, rapid muscle contractions that generate heat when your core temperature starts to fall. It can also kick in during a fever, after surgery, or as a side effect of certain drugs, which means the sensation of uncontrollable trembling does not always point to the same cause. Most episodes are harmless and stop on their own once you warm up or your fever breaks, but a few patterns of shivering deserve prompt medical attention.
How Your Brain Decides You Need to Shiver
Cold receptors in your skin send signals to a region of the brain called the hypothalamus, which acts as the body’s thermostat. When incoming signals indicate that skin temperature is dropping, neurons in a small area called the median preoptic nucleus kick off a chain of defensive responses. Animal research has shown that this area is essential for launching shivering: when it is chemically silenced, skin cooling fails to produce any of the normal heat-generating or heart-rate responses.1PubMed Central. Central efferent pathways for cold-defensive and febrile shivering The process works through a relay of inhibitory signals that ultimately release the brakes on motor neurons controlling skeletal muscle, causing the rapid, rhythmic contractions you feel as shivering.
Shivering is not the only trick the body has. You also produce heat through non-shivering thermogenesis, which relies on brown adipose tissue, a specialized fat that burns calories to generate warmth directly.2Diabetes & Metabolism Journal. Brown Fat as a Regulator of Systemic Metabolism beyond Thermogenesis In practice, both systems operate in parallel during cold exposure, but shivering is the louder, more noticeable one because it hijacks your muscles in a way you cannot override with willpower.
Why You Shiver During a Fever
It seems contradictory: your body temperature is climbing, yet you feel freezing and cannot stop shaking. The explanation is that during an infection, inflammatory molecules effectively turn the thermostat up. Your hypothalamus now “believes” your normal temperature is too low, so it deploys the same cold-defense toolkit it would use on a winter day. Specifically, a signaling molecule called prostaglandin E2 acts on receptor-expressing neurons in the brainstem that normally relay cold sensations. Research in animal models has demonstrated that this molecule amplifies cold-sensory transmission, producing the unpleasant chills people describe during febrile illness.3The Journal of Physiology. The pyrogenic mediator prostaglandin E2 elicits warmth seeking via EP3 receptor-expressing parabrachial neurons: a potential mechanism of chills
The practical result is that you feel cold and seek warmth even though a thermometer would show you already have a temperature of, say, 39 °C. The shivering generates extra heat to push your core temperature even higher, toward the new “set point” your brain has dialed in. Once the infection is under control and those inflammatory signals drop off, the thermostat resets downward, you start sweating, and the chills stop.
Shaking Chills as a Warning Sign
Not all fever-related shivering is equal. Most viral infections produce mild chills, but intense, visible shaking, sometimes called rigors, can point to bacteria in the bloodstream. A systematic review and meta-analysis found that shaking chills are a highly specific sign of bacteremia, meaning that when a patient with a fever is visibly shaking and trembling, the odds that bacteria have entered the blood rise considerably.4PubMed Central. Utility of shaking chills as a diagnostic sign for bacteremia in adults: a systematic review and meta-analysis This does not mean every bout of shivering during the flu is dangerous, but rigors severe enough that your teeth chatter uncontrollably and your bed shakes deserve a call to a doctor, especially if accompanied by a high fever, rapid heart rate, or confusion.
Clinicians pay close attention to the severity and timing of chills. Mild chills during a cold are expected; intense rigors in someone with an indwelling catheter, a recent surgical wound, or a compromised immune system are treated as red flags warranting blood cultures and sometimes empiric antibiotics before results come back.
Hypothermia and the Danger of Losing Your Shiver
When your core temperature drops below about 35 °C, you are in hypothermia territory. Shivering is vigorous at first, because the body is throwing everything it has at the problem. But as core temperature continues to fall, the muscles fatigue and the brain’s signaling becomes sluggish. Below roughly 32 °C, shivering often stops entirely. Paradoxically, this is the most dangerous phase: the absence of shivering in someone who is clearly cold suggests their body has lost the ability to rewarm itself. If you come across a person outdoors in freezing conditions who is confused, clumsy, and no longer shivering, they need emergency medical care immediately.
Rewarming a hypothermic person requires care. Aggressive external warming can cause cold blood from the extremities to rush back to the heart, potentially triggering dangerous heart rhythms. This is why medical guidance emphasizes gentle, core-focused rewarming for moderate to severe cases, ideally in a hospital.5Medical Hypotheses. Hypothermia-rewarming: A Double-edged sword? For mild hypothermia where the person is still shivering and alert, moving them to a warm environment, removing wet clothing, and wrapping them in dry blankets is generally sufficient.
Babies, Older Adults, and the Shivering Gap
Newborns do not shiver. They lack the skeletal muscle mass needed to generate meaningful heat through muscle contractions. Instead, they rely heavily on brown adipose tissue, which converts stored energy directly into warmth through non-shivering thermogenesis.6PubMed. Brown Adipose Tissue in Human Infants This is one reason babies are bundled so carefully: they simply do not have the backup system adults take for granted. As children grow and gain muscle, shivering becomes the primary cold-defense response.
At the other end of life, older adults still shiver, but the threshold at which it kicks in is lower. A study comparing surgical patients under 80 with those 80 and older found that the older group did not begin shivering until their core temperature dropped to about 35.2 °C, compared with 36.1 °C for the younger group.7PubMed. Shivering threshold during spinal anesthesia is reduced in elderly patients That nearly one-degree difference means an elderly person’s body waits longer before sounding the alarm, leaving them more vulnerable to cold-related harm without realizing it. Combined with thinner skin, less subcutaneous fat, and reduced circulation in many older adults, this lowered threshold helps explain why hypothermia disproportionately affects the elderly, even indoors during a cold snap.
Medications, Drugs, and Withdrawal
Shivering can appear as a side effect of medications or as a symptom of withdrawal, and these episodes look different from cold-induced trembling.
Serotonin syndrome is one of the more alarming medication-related causes. It occurs when drugs that boost serotonin levels push the system into overdrive, overactivating receptors in the brain and peripheral nervous system. The result is a triad of mental-status changes, autonomic instability, and neuromuscular hyperactivity that includes tremor and shivering.8PubMed Central. Serotonin syndrome It can happen when someone starts a new antidepressant, combines two serotonin-boosting drugs, or overdoses on one. If you notice shivering alongside agitation, rapid pulse, dilated pupils, and muscle twitching shortly after a medication change, treat it as an emergency.
Alcohol withdrawal is another common trigger. When someone who has been drinking heavily for an extended period suddenly stops, the resulting chemical imbalance in the brain can produce tremors, irritability, anxiety, and in severe cases, seizures and delirium.9PubMed Central. Introduction to alcohol withdrawal The shaking and chills that accompany withdrawal are not caused by cold but by a nervous system that has adapted to the presence of a sedative and suddenly finds itself without it. Opioid withdrawal produces a similar picture, with shivering, goosebumps (the origin of the phrase “cold turkey”), sweating, and muscle aches.
Post-Anesthesia Shivering
Waking up from surgery to find yourself shaking uncontrollably is common enough that anesthesiologists plan for it. Postoperative shivering is one of the most frequent complications of anesthesia, and while hypothermia from the cool operating room is the usual culprit, it can also happen in patients whose temperature is normal. Pain and acute withdrawal from short-acting opioids used during the procedure are additional potential triggers.10PubMed Central. Postanaesthetic shivering – from pathophysiology to prevention For patients, it is uncomfortable and sometimes distressing, but it is generally manageable with warming blankets, small doses of medications like meperidine, and time. For the clinical team, it is worth managing because shivering after surgery increases oxygen consumption and can strain the heart in patients who are already recovering from a procedure.
What Shivering Costs Your Body
Shivering is expensive, metabolically speaking. Your muscles are contracting rapidly without doing any useful mechanical work, so all the energy goes to heat. Research measuring energy expenditure during controlled cold exposure found that even mild cold increased metabolic rate by about 50 to 60 percent above resting levels, and moderate cold with visible shivering roughly doubled it.11PubMed Central. Acute mild cold exposure with shivering reduces 24 h glucose levels in individuals with type 2 diabetes but not prediabetes That same study found that cold exposure with shivering reduced 24-hour blood glucose levels in people with type 2 diabetes, which has prompted interest in whether controlled cold exposure could be used as a metabolic therapy. The effect was not seen in people with prediabetes, suggesting that the metabolic response to shivering is not uniform across populations.
The fuel mix changes too. During mild cold, the body burns more fat. During heavier shivering, carbohydrate oxidation ramps up alongside fat burning to meet the sharply higher energy demand.11PubMed Central. Acute mild cold exposure with shivering reduces 24 h glucose levels in individuals with type 2 diabetes but not prediabetes This is why prolonged shivering leaves you feeling exhausted and hungry. Your body has been running at roughly double its baseline energy burn with no food intake to match.
Shivering as an Obstacle in Critical Care
In intensive care units, doctors sometimes deliberately cool patients after cardiac arrest or severe brain injury, a strategy called targeted temperature management. The idea is that lowering body temperature slows harmful chemical cascades in the brain. The problem is that the patient’s body does not know this is intentional and fights back with vigorous shivering, which can negate the cooling effort and increase metabolic demand on an already-stressed brain.12PubMed. Control of Shivering During Targeted Temperature Management Shivering during targeted temperature management can contribute to secondary brain injury, making it a serious clinical concern rather than a minor annoyance.13PubMed. Continuous surface EMG power reflects the metabolic cost of shivering during targeted temperature management after cardiac arrest
To suppress shivering in these patients, ICU teams use a stepwise approach: surface counter-warming (placing warm blankets on the hands and face while cooling the core), medications like buspirone and meperidine, and in severe cases, neuromuscular blocking agents that paralyze the muscles entirely. Monitoring the intensity of shivering through tools like surface electromyography helps clinicians balance enough sedation to prevent shivering without over-medicating a vulnerable patient.
Getting Used to the Cold
People who are regularly exposed to cold, whether through occupation, sport, or deliberate cold-water immersion, tend to shiver less over time. Research on cold adaptation has found that repeated short-term cold exposure lowers the temperature at which shivering begins, effectively widening the zone between the point where you start sweating and the point where you start shivering.14Semantic Scholar. The nature of human adaptation to cold Your sweating threshold stays the same, but the shivering threshold drops, so you tolerate a wider range of cool temperatures without a muscular response.
This is partly why outdoor swimmers and winter athletes report feeling less cold over a season of training. Their subjective comfort in cool environments improves, and objective measurements confirm that the shivering response is genuinely blunted. Whether this adaptation carries health risks is context-dependent. For a trained winter swimmer who knows their limits, it is a useful physiological adjustment. For an older adult who has become habituated to a cold home and no longer notices the chill, it can mask a slow slide into mild hypothermia.
Shivering in Other Species
Shivering is not unique to mammals. Many flying insects use a form of shivering thermogenesis to warm their flight muscles before takeoff. Bees and moths, for instance, vibrate their thoracic muscles rapidly while grounded, raising muscle temperature to the range needed for sustained flight. Birds do something functionally similar, though the details of their control mechanisms differ. The convergence of shivering-like heat generation across such distant branches of the animal kingdom underscores how fundamental the strategy is: when you need to be warm and your environment will not do it for you, burning fuel through rapid muscle activity is one of biology’s most reliable solutions.15PubMed. Control and regulatory mechanisms associated with thermogenesis in flying insects and birds
A Quick Guide to When Shivering Warrants Medical Attention
Most shivering resolves with warmth, rest, and time. But certain patterns should prompt you to seek care:
- Rigors with high fever: Intense, bed-shaking chills in the setting of a fever above 39 °C, especially with confusion, rapid breathing, or skin changes, can indicate bloodstream infection.
- Shivering that stops during cold exposure: If someone who was shaking in the cold suddenly becomes still, confused, and drowsy, they may be entering severe hypothermia and need emergency help.
- Shivering after a medication change: Tremor, agitation, and shivering that appear within hours or days of starting or combining serotonin-active drugs could signal serotonin syndrome.
- Prolonged shivering without obvious cause: Shaking that lasts more than an hour with no cold exposure, fever, or medication trigger is worth discussing with a doctor, as it can occasionally point to thyroid problems, low blood sugar, or other metabolic issues.
For ordinary cold-triggered shivering, the fix is straightforward: get to a warm environment, add layers, drink something warm, and give your body a few minutes to catch up. Your internal furnace is already doing exactly what it was designed to do.