Your body deliberately cools itself down as you fall asleep and continues adjusting its temperature throughout the night, which is why you sometimes kick off the covers at 2 a.m. only to pull them back an hour later. Core body temperature drops by roughly half a degree to a full degree Celsius during sleep, and different sleep stages handle heat regulation in distinct ways. That built-in thermostat cycling accounts for most of the hot-and-cold swings people notice, though hormones, medications, bedroom conditions, and certain medical issues can amplify the effect considerably.
Your Internal Clock Drives a Nightly Cooldown
The brain’s circadian clock, housed in a tiny region of the hypothalamus, orchestrates a daily rhythm in core body temperature. In mammals, sleep is consistently accompanied by a decline in core temperature, and the circadian system links this cooling process tightly to your arousal state.1PubMed. Body temperature and sleep The drop actually begins before you fall asleep. Core temperature starts declining in the hours leading up to bedtime, and the extent of that presleep reduction corresponds to how quickly you fall asleep and how well you sleep overall.2PubMed Central. Core body temperature changes before sleep are associated with nocturnal heart rate variability
This isn’t a passive process. Your body actively sheds heat by dilating blood vessels in the skin, especially in the hands and feet. That rush of warm blood to your extremities transfers heat from your core to the surface, where it radiates away. If you’ve ever noticed your feet feeling unusually warm right before you drift off, that’s the mechanism at work. Research has shown that the degree of blood vessel dilation in the hands and feet is actually the single best physiological predictor of how quickly someone falls asleep.3Nature. Warm feet promote the rapid onset of sleep The gradient between skin temperature at your extremities and skin temperature closer to your core outperforms other measures, including core temperature itself, heart rate, and even subjective sleepiness ratings, when it comes to predicting sleep onset.4PubMed. Functional link between distal vasodilation and sleep-onset latency?
This is also why a warm bath before bed can help you fall asleep faster, even though it sounds counterintuitive. The warm water dilates blood vessels in the skin, and once you step out, heat pours off your body rapidly. The result is an accelerated version of the natural presleep cooldown. Studies in older adults have confirmed that a hot-water bath before bed shortens the time it takes to fall asleep, and the proposed mechanism is exactly this vasodilation-driven heat loss.5PubMed Central. Hot-water bathing before bedtime and shorter sleep onset latency are accompanied by a higher distal-proximal skin temperature gradient in older adults
What Happens During REM Sleep
The temperature swings people feel most acutely tend to cluster around periods of REM sleep, which is the stage associated with vivid dreaming. During REM, your body’s thermoregulatory controls partially disengage. In animal studies, temperature regulation during REM sleep appears genuinely impaired: the brain heats up, and normal defenses like shivering and sweating are suppressed or absent.6PubMed Central. REM Sleep and Endothermy: Potential Sites and Mechanism of a Reciprocal Interference Brain temperature rises during REM in rodents because of shifts in blood flow patterns, and the amount of REM sleep that occurs is highly sensitive to ambient temperature, peaking near thermoneutral conditions and declining when the room is too cold or too warm.7PubMed Central. Why Do I Get Hot and Cold When Sleeping? – Section: Thermal theories of REM sleep
In humans, the picture is less dramatic but still noticeable. Sweating responses persist during REM sleep but are blunted compared to other stages.7PubMed Central. Why Do I Get Hot and Cold When Sleeping? – Section: Thermal theories of REM sleep Classic experiments exposing sleepers to different room temperatures found that skin temperature shows small but measurable increases during REM at warmer ambient temperatures, and that shivering, which readily occurs during wakefulness in a cold room, largely vanishes during sleep and only appears occasionally in lighter sleep stages at the lowest temperatures tested.8PubMed. Metabolism and thermoregulation during stages of sleep in humans exposed to heat and cold In practical terms, this means your body is less able to compensate during REM if the room is too hot or too cold, which is often when you wake up feeling uncomfortable.
REM periods get longer as the night progresses, which is why the worst temperature-related awakenings often happen in the second half of the night. Your core temperature is near its lowest point, your thermoregulatory system is in its least responsive mode, and any mismatch between your body’s heat output and the room or bedding environment becomes harder to correct automatically.
Why Sleep Evolved Alongside Cooling
The link between body cooling and sleep is not a human quirk; it runs deep across mammals. Whether a species is active during the day or the night, sleep onset consistently pairs with a decline in core temperature. Animals engage in thermoregulatory behaviors before sleeping, like building nests or curling up, and these behaviors create warm microclimates around the body. Researchers have argued that this is not just about comfort: direct skin warming can shorten the time it takes to fall asleep and specifically promotes the deeper, non-REM stages of sleep, likely through direct neuronal circuits rather than mere coziness.9Frontiers in Neuroscience. The Temperature Dependence of Sleep In other words, the experience of needing warmth around you while your core cools down is something mammals have been doing for a very long time. The hot-and-cold sensation at night may be, in part, what optimal sleep physiology actually feels like.
Hormonal Influences on Nighttime Temperature
Hormones are among the most powerful amplifiers of nighttime temperature swings, and two hormonal contexts stand out: the menstrual cycle and menopause.
Skin temperature during sleep fluctuates across the menstrual cycle. Tracking data from wearable sensors has shown that skin temperature during sleep is higher in the mid-luteal and late-luteal phases compared to the menstrual phase, and lower during ovulation than during menses.10International Journal of Women’s Health. Tracking Sleep, Temperature, Heart Rate, and Daily Symptoms Across the Menstrual Cycle with the Oura Ring in Healthy Women Progesterone, which rises after ovulation, is the main driver: it raises the body’s thermostat slightly, leading to warmer skin and a subjective sense of sleeping hotter. If you notice that your nighttime comfort seems to shift on a roughly monthly schedule, this hormonal rhythm is the likely explanation.
During menopause, the issue intensifies. Hot flushes and night sweats, the hallmark vasomotor symptoms of menopause, are caused by a narrowing of the thermoneutral zone in the brain. Normally, your body tolerates a range of core temperatures without triggering sweating or shivering. When estrogen levels drop during menopause, that comfortable range shrinks, so even small fluctuations in core temperature can trigger a full-blown sweating episode or a bout of chills. The narrowing is related to estrogen withdrawal but most likely involves changes in brain neurotransmitters rather than estrogen acting alone.11PubMed. Menopausal hot flushes and night sweats: where are we now? The result is the classic menopausal pattern: a sudden wave of heat, drenching sweat, and then feeling cold as the sweat evaporates.
Medications That Mess With Your Thermostat
If you recently started a new medication and your nighttime temperature comfort changed, the two events may be connected. Antidepressants are among the most common culprits. Excessive sweating affects roughly one in five patients taking antidepressants, and night sweats specifically are a well-recognized complaint.12PubMed Central. Managing Antidepressant-Induced Hyperhidrosis With Vitamin E: An Over-the-Counter Supplement-Based Approach SSRIs in particular may cause night sweats by blocking certain receptors and increasing noradrenergic activity, which ramps up the body’s fight-or-flight thermoregulatory pathways during sleep.13PubMed Central. Selective Serotonin Reuptake Inhibitors and Night Sweats in a Primary Care Population
Other drug classes that can disrupt nighttime temperature regulation include hormone therapies, certain blood pressure medications, steroids, and some diabetes drugs. Alcohol is worth mentioning here too, even though it’s not a prescription: it disrupts sleep patterns and biological rhythms, and the resulting rebound effects can include sweating and chills as the body metabolizes alcohol through the night.14PubMed Central. Alcohol hangover: mechanisms and mediators
Medical Conditions That Cause Night Sweats or Chills
When temperature swings during sleep are severe, persistent, or new, they sometimes point to an underlying medical issue worth investigating.
- Infections and fever: Any active infection can reset the brain’s thermostat. Pyrogenic cytokines trigger a cascade in the hypothalamus that raises the target temperature, causing vasoconstriction and shivering to generate heat, followed by sweating when the fever breaks.15Infectious Disease Clinics of North America. The Acute-Phase Response This cycle of chills-then-sweats is a classic sign that your immune system is fighting something.
- Thyroid disorders: Thyroid hormones directly modulate heat production by influencing metabolic rate and the activity of thermogenic pathways in muscle and fat tissue, and they also interact with the sympathetic nervous system’s control of body temperature.16Journal of Molecular Endocrinology. Effects of thyroid hormones on thermogenesis and energy partitioning An overactive thyroid tends to produce excess heat and night sweats, while an underactive thyroid can leave you feeling perpetually cold.
- Sleep apnea: Obstructive sleep apnea has been associated with night sweats, and the link is strong enough that unexplained nocturnal sweating is sometimes the symptom that leads to a sleep apnea diagnosis.17PubMed Central. Nocturnal diaphoresis secondary to mild obstructive sleep apnea in a patient with a history of two malignancies The repeated breathing interruptions and oxygen drops likely trigger sympathetic nervous system surges that produce sweating.
- Autonomic dysfunction: Conditions that affect the autonomic nervous system, including some connective tissue disorders like Ehlers-Danlos syndromes, can produce thermoregulatory dysfunction.18PubMed. Dysautonomia in the Ehlers-Danlos syndromes and hypermobility spectrum disorders-With a focus on the postural tachycardia syndrome If the wiring that controls blood vessel dilation, sweating, and shivering isn’t working properly, nighttime temperature regulation becomes erratic.
Persistent, drenching night sweats that soak your sheets are worth bringing up with a doctor, especially if accompanied by unexplained weight loss, fatigue, or other systemic symptoms. Most people who occasionally feel too hot or too cold during the night are experiencing normal sleep physiology, but the severity and pattern matter.
Your Bedroom Setup Matters More Than You Think
The microclimate between your skin and your bedding is where your body’s thermoregulatory efforts succeed or fail. Fabric choice has a measurable effect. In a controlled experiment comparing sleepwear and bedding materials at different room temperatures, wool sleepwear significantly shortened the time it took to fall asleep compared to cotton, with trends toward increased total sleep time and sleep efficiency as well.19PubMed Central. The effects of fabric for sleepwear and bedding on sleep at ambient temperatures of 17°C and 22°C Wool’s ability to buffer moisture and temperature likely helps maintain a more stable microclimate against the skin.
Room temperature itself plays a role, though the “ideal” number varies by person. What research consistently shows is that people are more sensitive to heat than cold during sleep: being too warm disrupts sleep more readily than being too cool. One study found that people’s thermal comfort temperature is actually higher during sleep than during wakefulness, meaning sleepers tolerate slightly warmer air than they prefer while awake.20Building and Environment. Experimental study on thermal comfort of sleeping people at different air temperatures Still, there’s a ceiling. A temperature-controlled mattress cover that cooled the bed surface during the first half of the night produced meaningful gains in deep sleep and REM sleep compared to sleeping without temperature control.21PubMed Central. Sleeping for One Week on a Temperature-Controlled Mattress Cover Improves Sleep and Cardiovascular Recovery
Layering your bedding so you can easily shed a layer in the night is one of the simplest interventions. A single adjustable blanket does less for you than a sheet plus a lighter cover, because you can remove one without fully waking up. Breathable fabrics and a room temperature somewhere around 18-20°C work for most people, though personal variation is real and experimenting is more useful than following a rigid number.
How Aging Changes the Equation
Older adults often report more trouble with nighttime temperature, and physiology backs this up. Body temperature tends to run lower with age, and the ability to tolerate thermal extremes declines. Some of this is structural: skin changes reduce blood flow and sweating capacity, and metabolic rate drops, limiting heat generation. These shifts directly impair the body’s ability to maintain thermal balance, particularly when challenged by a room that’s too cold or too warm.22Gerontology. Age-dependent changes in temperature regulation – a mini review
On top of that, older adults spend less time in deep sleep and more time in lighter stages, meaning they wake more easily when a temperature mismatch occurs. The combination of a less robust thermoregulatory system and more fragile sleep architecture means that bedroom environment becomes increasingly important with age. For older adults who find themselves frequently waking from temperature discomfort, the warm-bath strategy mentioned earlier has specific evidence supporting it: in older populations, a hot-water bath before bed reliably shortened sleep onset latency alongside the expected increase in the distal-proximal skin temperature gradient.5PubMed Central. Hot-water bathing before bedtime and shorter sleep onset latency are accompanied by a higher distal-proximal skin temperature gradient in older adults
Wearable Sensors and What They Can Tell You
The growing market of sleep-tracking wearables has introduced a new wrinkle: you can now see your skin temperature fluctuations graphed out night by night. Some devices track temperature at the finger, some at the wrist, and the data can reveal patterns you might not consciously notice, like a consistent temperature spike during the luteal phase or a gradual upward trend that correlates with getting sick. Researchers have begun using wearable-derived skin temperature data not just for sleep quality assessment but as a window into cardiovascular health. A recent study found that a physics-based model applied to overnight skin temperature and activity data could distinguish people with cardiovascular conditions affecting skin perfusion from healthy controls.23npj Digital Medicine. Wearable-derived skin temperature dynamics during sleep reveal cardiovascular perfusion deficits through mechanistic modeling
For the average person, the practical takeaway is more modest: if your wearable shows consistently elevated nighttime skin temperature, it could mean your room is too warm, you’re in the luteal phase, you’re fighting off an illness, or your bedding is trapping too much heat. The data is most useful when you track it alongside changes in your environment or health, turning a vague “I slept badly” into a specific “my temperature was elevated all night, and the room was three degrees warmer than usual.” It won’t replace a doctor’s evaluation for persistent problems, but it can help you figure out which nights are worth worrying about and which are just your circadian clock doing its job.