Why Do I Get Cold Chills When Falling Asleep?

Your body is deliberately cooling itself down. As you drift toward sleep, your core temperature drops, and the process your brain uses to make that happen can produce the shivery, goosebump-raising sensation you feel under the covers. This is not a sign that something is wrong. It is your internal thermostat doing exactly what it is supposed to do: shedding heat so you can fall asleep. The mechanism behind it, though, is more involved than most people realize, and several factors can make the sensation more or less noticeable from one person to the next.

Your Body Dumps Heat Right Before Sleep

Sleep in mammals comes with a built-in temperature drop. Your circadian clock, housed in a small region of the brain called the suprachiasmatic nucleus, orchestrates daily rhythms in both body temperature and wakefulness, keeping the two tightly linked.¹ As evening approaches, the clock shifts your thermostat downward, and your body starts losing heat. In studies where people were allowed to choose their own bedtime freely, they consistently chose the moment when their body temperature was falling at its fastest rate.² You do not wait for sleep to cool off. You cool off so that sleep can begin.

The primary way your body sheds this heat is through the skin, especially the hands and feet. Blood vessels in those areas widen, a process called vasodilation, flooding the skin’s surface with warm blood. That warm blood releases heat into the surrounding air, pulling thermal energy out of your core. Research has shown that the degree of blood vessel dilation in the hands and feet is the single best physiological predictor of how quickly someone falls asleep.³ This is why your feet sometimes feel warm or flushed when you first climb into bed, and it is also why, paradoxically, that rush of heat to the surface can leave the rest of you feeling chilly.

This vasodilation does not wait until lights-off. It ramps up as much as two hours before your usual sleep time, well before you are actually in bed. As core temperature falls, self-assessed alertness drops with it, and heart rate follows the same downward curve.² The cold chill you feel is essentially the subjective experience of being caught in the steepest part of that decline. Your skin is radiating heat outward; your core is losing temperature; and the gap between how warm your body was a few minutes ago and how warm it is now is large enough that your nervous system registers it as a chill.

Why Your Brain Ties Temperature to Sleepiness

The connection between cooling down and falling asleep is not accidental. It is wired into the brain at a deep level. The preoptic area of the hypothalamus contains neurons that respond to warmth detected at the skin’s surface. When your skin senses warmth, these neurons activate pathways that simultaneously promote the earliest stage of sleep and push core body temperature lower.⁴ This is why a warm bath before bed makes you sleepy: the heat at the skin triggers the brain to accelerate its cooling program, and sleep follows the temperature drop.

The coupling makes functional sense. One hypothesis is that lowering core temperature during sleep conserves energy. Your metabolic rate decreases, your heart slows, and less fuel is burned maintaining a high body temperature during hours when you are not active anyway. By linking sleep onset to the temperature decline, the brain ensures that you are already losing heat by the time you actually close your eyes, rather than spending the first hour of sleep catching up.

This tight coupling also explains why disrupting temperature can disrupt sleep. If your bedroom is too warm, the body cannot shed heat effectively, and the brain’s sleep-promoting circuits do not get the cooling signal they need. If the room is too cold, the body may fight to conserve heat rather than release it, which can also interfere with the process. The chill you feel when drifting off sits right at the sweet spot: your body is actively dumping heat, the temperature is falling fast, and you notice it as a brief wave of cold before sleep takes over.

What Happens to Your Heart and Nervous System

The temperature drop is not happening in isolation. As core body temperature declines before sleep, your autonomic nervous system shifts gears. The parasympathetic branch, sometimes called the “rest and digest” system, ramps up. Heart rate variability increases as the body transitions away from the alert, sympathetic-dominant state that characterizes waking hours.⁵ This means your heart rate slows, your blood pressure dips, and the body enters a calmer physiological mode all at once.

This cascade matters for understanding the chill, because the autonomic shift can amplify the sensation. When your sympathetic nervous system pulls back, blood vessels in the skin dilate more freely, speeding up heat loss. At the same time, the reduced metabolic activity means less internal heat is being generated to replace what is being lost. The result is a brief window where your body is cooling rapidly but has not yet fully adjusted to the new lower set point, and that window is exactly when people report feeling chilly, shivery, or goosebumpy as they lie in bed.

Individual differences in how sharply the autonomic system shifts may explain why some people consistently notice this chill and others rarely do. Research has found that the magnitude of the presleep temperature decline and the corresponding change in heart rate variability vary from person to person.⁵ If your body drops temperature steeply and quickly, the chill will be more noticeable. If the decline is more gradual, you may barely feel it.

Hormonal Factors That Amplify the Effect

For some people, the bedtime chill is not just a subtle shiver but a pronounced wave of cold, sometimes followed by a flush of heat. Hormonal changes can make the body’s temperature control less precise, turning normal fluctuations into exaggerated swings. The most well-studied example involves the drop in estrogen that occurs during perimenopause and menopause. Estrogen plays a role in maintaining the thermoregulatory set point in the hypothalamus, and as levels fall, the body’s tolerance for temperature shifts narrows dramatically. Small changes in core temperature that would normally go unnoticed instead trigger full heat-loss responses, including vasodilation, sweating, and chills.⁶

These vasomotor symptoms, commonly experienced as hot flashes and night sweats, can also show up as chills. The pattern often follows a recognizable sequence: a sudden flush of warmth, then sweating, then a chill as the evaporating sweat rapidly cools the skin. Because the body is already in its presleep cooling phase, the chill component can be especially sharp at bedtime. If you are in your 40s or 50s and noticing that bedtime chills are newer or more intense than they used to be, fluctuating hormone levels are a likely contributor.

Thyroid hormones also play a role in baseline temperature regulation. An underactive thyroid slows metabolism and reduces the body’s ability to generate heat, which can make the normal presleep temperature drop feel more dramatic. This is distinct from the circadian cooling mechanism itself; the body is still doing the same thing it always does, but the starting temperature is lower and the reserves for warming back up are smaller. People with untreated hypothyroidism often report feeling cold in general, and the presleep window can be the time they notice it most.

How Your Bedroom Changes the Experience

The environment you sleep in can make the presleep chill feel mild or miserable. Your body needs to lose heat to fall asleep, and the surrounding air temperature determines how efficiently that happens. In a room that is too warm, heat loss is sluggish, and sleep onset may be delayed because the cooling signal never gets strong enough. In a room that is too cool, the body can lose heat faster than the circadian program intended, and the chill becomes more pronounced.

Research on mild cold stress during sleep illustrates the effect. When women slept without bedcovers in a warm room (around 27 to 28°C), their core body temperature and the low point of their circadian rhythm both dropped significantly compared to baseline nights with covers.⁷ The body compensated for the extra heat loss, but the process was measurably different from a normal night. Even a room temperature that feels comfortable while you are awake can feel cold during the presleep period, because your body is already shedding heat on its own schedule. The bedcovers and pajamas you choose essentially set the thermal microclimate around your skin, and getting that microclimate wrong in either direction affects both the chill and the quality of your sleep.

Interestingly, the study of presleep behavior in normal subjects found that people naturally gravitate toward a moderate thermal environment before bed. Environmental temperature ratings in the pre-sleep period clustered right at the middle of the scale, neither too hot nor too cold, and the researchers suggested that a constant, mild environment with limited physical activity may predispose the body to the thermoregulatory changes that precede sleep onset.⁸ In other words, people instinctively seek out conditions that support the cooling process. The chill is more likely to catch your attention when something disrupts that instinct: a room that is drafty, a blanket that is too thin, or a heating system that cycles off at the wrong time.

When the Chill Might Mean Something Else

Most bedtime chills are the benign result of the cooling process described above. But chills that are new, severe, or accompanied by other symptoms deserve a closer look. A fever from an infection can produce chills at any time of day, and because people are often lying still and paying attention to their body at bedtime, that may be when they first notice them. If your chills come with body aches, sweating that soaks the sheets, or a temperature above your normal baseline, an infection or inflammatory condition is worth considering.

Anemia reduces the blood’s capacity to carry oxygen and can impair heat generation, making the normal presleep temperature drop feel more severe. Iron-deficiency anemia is common enough, particularly in menstruating women and older adults, that it is worth ruling out if bedtime chills are a persistent issue. Similarly, blood sugar fluctuations in poorly controlled diabetes can affect thermoregulation, and some medications, including certain antidepressants and blood pressure drugs, list chills or temperature sensitivity as side effects.

Restless legs syndrome (RLS) is another condition that overlaps with bedtime discomfort in ways that can confuse the picture. People with RLS often experience uncomfortable sensations in their legs precisely when they lie down to sleep, and research has found that a majority of RLS patients have impaired temperature perception in their limbs.⁹ If your bedtime chill comes with an irresistible urge to move your legs, or if the cold sensation is localized to your lower limbs rather than generalized, RLS is worth discussing with a doctor.

Shivering Fades Once You Are Asleep

One of the more curious aspects of presleep chills is that they tend to disappear once you actually fall asleep. This is not just because you stop noticing them. Research on thermoregulation across sleep stages has shown that shivering, which is the body’s primary defense against cold, is actively suppressed during sleep. In experiments where people slept in cool environments (around 21°C), shivering was common during wakefulness but occurred only occasionally during the lightest stages of sleep and essentially vanished during deeper stages.¹⁰ The brain appears to relax its defense of a fixed core temperature during sleep, allowing the body to settle at a lower set point without triggering corrective responses like shivering.

This explains why the chill is so specific to the transition period. While you are awake, your brain detects the falling temperature and responds with shivering and goosebumps, the classic cold-chill feeling. Once sleep takes over, those responses are dialed back, and the lower temperature is accepted as the new normal for the night. You are only caught in the uncomfortable middle zone for the few minutes it takes to actually fall asleep. If you find yourself lying awake and shivering for a long time, the issue is more likely that you are too cold to fall asleep, not that the normal presleep cooling is causing prolonged discomfort.

An Ancient Link Between Sleep and Saving Energy

The connection between sleep and dropping body temperature is not a quirk of human physiology. It appears to be one of the oldest features of sleep in warm-blooded animals. Across species, longer or deeper sleep is associated with greater drops in body temperature, and the spectrum from light sleep to deep sleep to torpor to full hibernation forms a continuous gradient of temperature reduction and energy savings.¹¹ The hypothesis is that sleep evolved alongside the ability to maintain a constant body temperature, in part as a strategy to offset the enormous metabolic cost of being warm-blooded.

Maintaining a core temperature well above the surrounding environment is expensive. It demands a constant supply of calories and a high resting metabolic rate. Sleep offers a daily window to dial that cost down. By lowering the set point for a few hours, the body saves energy that would otherwise go toward maintaining a temperature nobody needs during a period of inactivity. When energy stores drop, some animals respond by lowering body temperature even further during sleep, or by sleeping longer.¹¹ Humans do not hibernate, but the nightly dip in temperature follows the same fundamental logic: sleep is when the body resets the thermostat to save fuel.

From this perspective, the chill you feel while falling asleep is the sensation of your body entering its nightly energy-conservation mode. The shiver is a brief protest from your waking thermostat before the sleeping thermostat takes over and accepts the lower setting. It is an ancient trade-off, one that warm-blooded animals have been making for millions of years, that just happens to be noticeable in the few quiet minutes before you lose consciousness.

Practical Ways to Manage It

If presleep chills bother you, the most effective approach targets the mechanism directly. Warming your extremities before bed accelerates the vasodilation process that your body is already trying to initiate. Wearing socks to bed or using a warm foot bath in the hour before sleep can help the hands and feet dump heat earlier, so the steepest part of the temperature decline happens while you are still comfortably active rather than lying under the covers feeling cold. The research showing that warm feet predict faster sleep onset supports this approach on physiological grounds, not just as a comfort measure.³

Bedroom temperature matters, but not in the way most people assume. A cooler room (typically in the range of 18 to 20°C) is generally better for sleep than a warm one, because it supports the heat-loss process. The trick is to keep the air cool while keeping your skin insulated enough to prevent the kind of active shivering that keeps you awake. A breathable blanket that traps enough warmth to prevent chills without causing overheating is more useful than cranking the thermostat. Layering bedding so you can kick off a top layer if you get warm later in the night is a simple adjustment that respects the body’s shifting needs across the sleep cycle.

Timing also plays a role. Because the presleep temperature decline begins up to two hours before your typical sleep time, getting into bed much earlier than usual can put you in a cold room before the cooling process has started, making you feel genuinely cold rather than experiencing the normal chill of transition. Keeping a consistent sleep schedule allows the circadian clock to coordinate the temperature drop with your actual bedtime, reducing the mismatch. A warm shower or bath about 60 to 90 minutes before bed exploits the same principle: it warms the skin, triggers vasodilation, and the subsequent rapid heat loss aligns the cooling curve with sleep onset, smoothing the transition so the chill is briefer and less noticeable.