Waking up shivering usually means your body’s core temperature has dipped below its comfort range and your muscles are contracting rapidly to generate heat. The trigger can be as simple as a cold bedroom or a kicked-off blanket, but it can also point to an underlying process like an infection, a blood sugar drop, a hormonal shift, or a thyroid condition. Because your body’s temperature-regulation system actually weakens during certain stages of sleep, you are more vulnerable to chills at night than you might expect.
How Your Body Controls Temperature During Sleep
Shivering is one of your body’s fastest tools for warming up. When sensors in your skin detect cold, a region deep in the brain coordinates a cascade of responses: blood vessels near the skin constrict to keep warm blood closer to your organs, brown fat tissue burns calories to produce heat, and skeletal muscles begin involuntary contractions. Those contractions are shivering, and they can roughly double your resting heat production within minutes. Research in animal models shows that when the brain area responsible for detecting the cold signal is chemically silenced, shivering and the accompanying rise in heart rate stop entirely, confirming that the whole response is centrally orchestrated rather than a reflex happening at the muscle level alone.1PubMed Central. Central efferent pathways for cold-defensive and febrile shivering
During sleep, this system does not operate at full strength. Your core temperature naturally falls by roughly half a degree Celsius over the course of the night as part of the circadian cycle, reaching its lowest point in the early-morning hours. That dip is normal and even helps you stay asleep. But during REM sleep, the stage associated with vivid dreaming, your thermoregulation becomes significantly impaired. Your body largely stops shivering and sweating in REM, behaving almost as if it has temporarily given up on maintaining a fixed temperature.2PubMed Central. REM Sleep and Endothermy: Potential Sites and Mechanism of a Reciprocal Interference If your sleeping environment cools down during one of these REM episodes, your body may not mount a shivering response until the episode ends. That delayed response can produce a sudden wave of shivering intense enough to wake you.
When the Room Is Simply Too Cold
The most common reason for waking up shivering is an environment that has dropped below the range your body can compensate for while sleeping. Research on sleep and ambient temperature finds that cold exposure leads to more sleep disruptions than heat exposure when a person is not well insulated by blankets or clothing.3ScienceDirect. The importance of temperature and thermoregulation for optimal human sleep That is partly because the adaptable range of temperatures for sleeping people is narrower on the cold side. With adequate blankets the picture reverses, since insulation protects against cold better than it protects against heat, but many people underestimate how much their bedroom cools overnight, especially in transitional seasons when heating may not be running.
A few scenarios reliably set you up for a cold-triggered wake-up. Air conditioning set on a timer that keeps running after you fall asleep, an open window that lets in a pre-dawn chill, or simply kicking off your covers during a restless dream can each push your skin temperature down enough to trigger shivering. Because your core temperature is already at its nightly low in the hours before dawn, any additional environmental cooling stacks on top of an already reduced baseline. The fix is straightforward: keep the bedroom between roughly 18 and 22 degrees Celsius, use layered bedding you can adjust without fully waking, and avoid sleeping directly in the path of a fan or air vent.
Fever and Infection
Shivering that wakes you up alongside a feeling of being generally unwell, achy, or flushed often signals that your body is fighting an infection. During a fever, the brain resets its target temperature upward. Until the body catches up to the new, higher set point, you feel cold even though your actual temperature is climbing. Shivering is the mechanism that closes the gap: your muscles generate heat until your core temperature reaches the new target, at which point the shivering stops and you start to feel warm, or even overheated.
A routine viral illness like the flu or a common cold is the most likely culprit. These fevers tend to peak in the evening and early night, so the shivering-and-chills phase often lines up with bedtime or the early hours of sleep. If the fever breaks overnight, you may go through the reverse cycle: sweating and feeling hot as your set point drops back toward normal. Waking up drenched in sweat after an episode of shivering is a classic pattern during acute infections.
Low Blood Sugar in the Night
A drop in blood sugar, known as nocturnal hypoglycemia, can produce shivering, sweating, and a racing heart during sleep. When blood glucose falls, the body releases stress hormones like adrenaline to mobilize stored energy and raise the glucose level back up. Those same stress hormones trigger sweating and peripheral vasodilation, which opens blood vessels near the skin. The result is a paradox: you are sweating and losing heat through your skin even as your core temperature falls. One study found that insulin-induced hypoglycemia caused core temperature to drop by about half a degree Celsius and skin temperature to fall even further, driven by the sweating and vasodilation that accompany the low blood sugar.4Portland Press. Hypoglycaemia, Hypothermia and Shivering in Man Shivering then kicks in as the body tries to recover that lost heat.
This matters most for people with type 1 diabetes, who are vulnerable to overnight lows from insulin dosing. Research shows that their bodies mount a weaker hormonal response to hypoglycemia during sleep compared with waking hours, with adrenaline levels during sleep-time lows reaching only about a third of what they reach when the same glucose level is experienced while awake.5Diabetes. Sleep-Related Hypoglycemia-Associated Autonomic Failure in Type 1 Diabetes: Reduced Awakening From Sleep During Hypoglycemia That blunted response means people with diabetes are less likely to wake up and correct the low, making nighttime hypoglycemia both harder to detect and more dangerous. Healthy people without diabetes rarely experience blood sugar drops severe enough to cause overnight shivering, but it can happen after heavy alcohol consumption, prolonged fasting, or extreme exercise late in the day.
Thyroid Conditions and Feeling Perpetually Cold
If you wake up shivering frequently and also feel cold during the day, an underactive thyroid could be involved. Thyroid hormones set the pace of your metabolism, and when levels are low, your body generates less heat at baseline. A study of people with spontaneous hypothyroidism found that their cold-induced thermogenesis, the body’s ability to ramp up heat production when exposed to cold, was significantly reduced compared with healthy controls. Once thyroid hormone replacement restored normal levels, cold-induced heat generation increased substantially.6PubMed Central. Resolution of Hypothyroidism Restores Cold-Induced Thermogenesis in Humans
The practical consequence is that the same bedroom temperature that feels perfectly comfortable to someone with a normal thyroid may be genuinely too cold for someone with untreated hypothyroidism. Their body simply cannot generate enough heat to compensate. Other symptoms typically accompany hypothyroidism: fatigue, weight gain, dry skin, and constipation are common. If shivering at night is part of a broader pattern of cold intolerance and low energy, it is worth having thyroid function checked with a simple blood test.
Hormonal Shifts and the Thermoneutral Zone
Perimenopause and menopause are notorious for disrupting sleep with hot flashes and night sweats, but the aftermath of a hot flash often includes shivering. The underlying issue is a narrowing of the thermoneutral zone, the range of core body temperatures your brain considers normal. In people going through estrogen withdrawal, that zone shrinks, so even tiny fluctuations in core temperature that would normally go unnoticed trigger a full vasomotor response: blood vessels suddenly dilate, you flush and sweat, and then the rapid heat loss from all that sweating overshoots, leaving you cold and shivering.7PubMed. Menopausal hot flushes and night sweats: where are we now? The hot flash itself may wake you, or the shivering that follows it might.
This cycle can repeat multiple times per night during the worst of menopausal symptoms, fragmenting sleep severely. It is not limited to people going through natural menopause; anyone undergoing a sudden drop in estrogen, such as after surgical removal of the ovaries or during certain cancer treatments, can experience the same narrowing of the thermoneutral zone. The shivering in these cases is not a sign of illness but rather an exaggerated thermoregulatory overcorrection.
Alcohol and Substance Withdrawal
Alcohol has a strong effect on peripheral blood flow. Drinking before bed dilates blood vessels near the skin, which is why you feel warm initially, but that same vasodilation radiates heat away from your core. As your body metabolizes the alcohol over the course of the night, blood vessels constrict again, and the resulting temperature swings can trigger shivering. Heavy drinkers face a more intense version of this problem during withdrawal. Monitoring of people going through alcohol withdrawal has shown that sweating peaks in the hours between midnight and 6 a.m., with skin temperature also elevated during that window.8PubMed. Twenty-four hour ambulatory monitoring of tremor, sweating, skin temperature and locomotor activity in the alcohol withdrawal syndrome That profuse sweating, combined with the autonomic instability of withdrawal, can produce cycles of drenching sweat followed by chills and shivering that are distinct from ordinary cold exposure.
Certain medications can also alter thermoregulation in ways that lead to overnight shivering. Opioid withdrawal is well known for producing chills and goosebumps (the origin of the phrase “cold turkey”). Some antidepressants, particularly SSRIs, can cause night sweats that leave you cold as the sweat evaporates. Anesthesia and sedation drugs sometimes lower the body’s shivering threshold, which is why post-operative shivering is so common in hospital recovery rooms. If you notice shivering starting or worsening after beginning a new medication, that connection is worth discussing with your prescriber.
When Shaking Chills Point to Something Serious
Most overnight shivering is benign, but there is a specific pattern that warrants prompt medical attention: rigors, or shaking chills so intense that your whole body trembles uncontrollably. Rigors accompanied by high fever can be a sign of bacteremia, meaning bacteria have entered the bloodstream. A systematic review found that shaking chills are highly specific for bacteremia, meaning they do not often occur without it, though they are not especially sensitive, meaning plenty of people with bacteremia never experience rigors.9PubMed Central. Utility of shaking chills as a diagnostic sign for bacteremia in adults: a systematic review and meta-analysis In clinical terms, when a patient reports shaking chills rather than just feeling cold, the likelihood of bacteremia roughly quadruples.10JAMA. Does This Adult Patient With Suspected Bacteremia Require Blood Cultures?
The significance of rigors appears even stronger in older adults. A study separating patients by age found that shaking chills were a stronger predictor of bacteremia in people over 80 than in younger adults, with roughly triple the odds of a positive blood culture when rigors were present.11PubMed Central. Shaking chills and high body temperature predict bacteremia especially among elderly patients Bacteremia can escalate to sepsis rapidly, so this is not a wait-and-see situation. If you or someone in your household wakes up with violent, full-body shaking chills and a high fever, especially combined with confusion, rapid breathing, or feeling much sicker than a typical cold would explain, seek medical care urgently.
Brown Fat, Cold Exposure, and Why Some People Shiver More Than Others
Not everyone shivers at the same temperature. Part of the reason is variation in brown adipose tissue, a type of fat that burns calories to produce heat without requiring you to shiver. When healthy adults are exposed to mild cold in the range of 16 to 19 degrees Celsius, their daily energy expenditure rises by roughly 190 kilocalories on average, and both the volume and activity of brown fat increase measurably.12PubMed Central. Effect of Acute Cold Exposure on Energy Metabolism and Activity of Brown Adipose Tissue in Humans: A Systematic Review and Meta-Analysis People with more active brown fat can maintain their core temperature through this non-shivering heat production alone, while those with less brown fat rely more heavily on shivering to stay warm.
Brown fat activity tends to be higher in younger people, in leaner people, and in those who are regularly exposed to cool environments. It declines with age, with obesity, and in people who spend most of their time in tightly climate-controlled settings. This helps explain why some people seem to wake up shivering under conditions that their partner sleeps through comfortably: the two bodies may have very different capacities for generating heat without shivering. It also explains why older adults are especially prone to waking up cold. Their brown fat reserves are lower, their shivering threshold is higher, and their vascular responses are slower, all of which add up to less effective overnight temperature regulation.
Practical Patterns Worth Paying Attention To
A single episode of waking up shivering almost never means anything worrisome. The blanket slipped off, the window was open, or you are coming down with a cold. The pattern matters more than any individual episode. Frequent overnight shivering despite a warm room and adequate bedding suggests something systemic: a thyroid issue, recurrent nocturnal hypoglycemia, hormonal changes, or a medication side effect. Keeping a brief log of when the shivering happens, what the room conditions are, and whether any other symptoms accompany it can save time when you bring it up with a doctor.
A few red flags justify faster action. Shivering accompanied by a fever above 38.5 degrees Celsius, especially if it includes rigors, warrants medical evaluation the same day. Shivering with confusion, rapid pulse, or difficulty breathing needs emergency attention. Recurrent overnight shivering in someone with type 1 diabetes should prompt a review of nighttime glucose data, since the blunted arousal response during sleep-time hypoglycemia means the shivering might be happening at dangerously low blood sugar levels without reliably waking the person up.5Diabetes. Sleep-Related Hypoglycemia-Associated Autonomic Failure in Type 1 Diabetes: Reduced Awakening From Sleep During Hypoglycemia
Why Early Morning Is the Most Vulnerable Window
If you notice that your shivering tends to happen in the last few hours before your alarm, that is not a coincidence. Several factors converge in the pre-dawn hours to make you maximally susceptible to feeling cold. Your core temperature is at its circadian low point. REM sleep episodes become longer and more frequent as the night progresses, meaning you spend more time in the sleep stage where thermoregulation is weakest.2PubMed Central. REM Sleep and Endothermy: Potential Sites and Mechanism of a Reciprocal Interference Outdoor temperatures are also at their lowest, which means your bedroom has had all night to cool down. And if you had alcohol or a large meal in the evening, the metabolic effects of processing those have largely worn off, removing any extra heat generation they provided earlier in the night.
For people whose shivering consistently clusters in the 4 to 6 a.m. window, the simplest interventions target this timing directly. A programmable thermostat set to bump up the heat an hour before you normally wake, an extra blanket kept folded at the foot of the bed for easy grabbing, or warmer sleepwear on the upper body (where heat loss through radiation is greatest) can all break the pattern without requiring any medical investigation. If those environmental fixes do not resolve the problem, the shivering is likely driven by something internal rather than something external, and that is when a conversation with your doctor becomes worthwhile.