Written by Jill Zwarensteyn, Editor
If you have a remote-controlled fan plugged into the end of your bed, or if cooling properties were the main factor influencing your last mattress purchase, or even if you sleep with the window open in the dead of winter, you’ve probably accepted that you sleep hot. But have you ever asked yourself why? How the body regulates temperature at night can have a huge effect on your quality of sleep.
When you struggle with your bedding and pillows to find constantly find the sole cool spot on the bed, your sleep deprivation can influence your body’s ability to regulate your temperature cycle, and in time your inability to regulate temperature can lead to more insomnia. If you’re looking for a way to break the cycle or just learn more about your body, this article should help point you in the right direction.
Why Do We Sweat and Shiver?
The short answer is that our bodies are good at controlling temperature, and when they detect the normal methods aren’t doing a good enough job, they’ll hit the override switch to quickly cool or warm us up. Heat regulation is an essential characteristic, and studies show thermoregulation keeps us alive when temps plunge or rise unexpectedly.
According to a study at Harvard University, our bodies have two main ways of regulating temperature, including behavioral and physiological mechanisms. When we are cold, we may shiver in an attempt to warm up via rapid muscle contractions. While shivering usually lets us know we should head inside or put on a jacket, it’s also incredibly effective on its own, accounting for heat production up to five times the amount produced at rest, according to the study.
Similarly, when we are too hot, we usually begin to sweat, bringing moisture to the surface of the skin. When this water evaporates, there is a cooling effect that is pretty effective at lowering body temperature.
Explaining the Biology
While sweating is supposed to cool you off, when was the last time you started sweating and then noticed you were feeling cooler? Probably not recently.
According to Harvard Medical School, this is because thermoregulation isn’t as much about comfort as it is staying alive. Think of how awful you feel when you have a fever. That is only the difference of a few degrees, with a wildly uncomfortable result. What would happen if your temperature always fluctuated with external temperatures?
Our bodies need to be in a specific temperature range to function properly, usually about 98.6°F. To keep our bodies in homeostasis, or stable body temperature, our bodies mostly use vasomotor control, which is just a fancy term for mechanisms of heat regulation via the circulatory system.
When we need to cool down, vasodilation radiates heat from the blood vessels to outer tissues, resulting in sweating and feeling warm at the skin level. With vasodilation during exercise, blood flow can increase up to ten times in volume, allowing you to stay at a stable body temperature even if you are sweating profusely.
To retain heat, vasoconstriction restricts blood vessels to stop them from transferring heat, effectively maintaining body temperature. However, you may feel your hands and feet getting colder as blood is drawn away from the surface of the skin and extremities. So while sweating and shivering may not make you feel any better at the surface level, they are still vital functions in maintaining core body temperature and keeping you in homeostasis.
Find Out More: Causes of Night Sweats
Tracking Temp Throughout Your Circadian Rhythm
So what does this have to do with sleep? Thermoregulation isn’t just about keeping you alive, it is also closely tied to the circadian rhythm, both creating the optimal resting temperature when timed correctly and signaling the brain when it’s time to rest.
According to a German study, falling asleep is accompanied by a drop in core body temperature, whether you rest during the day or the night. When you go into non-rapid eye-movement periods, the brain also cools.
When these internal temperatures begin to steeply decline, we are likely to engage in nesting behaviors that lead us to choose sleep, i.e. getting a blanket, lying down, breathing steadily. When we dissociate from this pattern, we generally experience a harder time falling asleep, according to an Australian study.
Because our circadian rhythm is so closely tied to the first episode of NREM each night, studies show that when we experience sleep deprivation, the circadian temperature rhythm can become disrupted. Essentially this means our body clocks get out of sync, meaning they don’t cool us down at the right time to signal to our brain it’s time to rest. This can lead to a variety of disorders, but most commonly it can result in insomnia.
This theory is supported by evidence suggesting those with vasospastic disorders, or disorders affecting one’s ability to restrict and expand veins and arteries to maintain or release heat, tend to experience more sleep deprivation problems than those without the disorder.
Many of these issues have to do with sleep propensity or the body’s readiness to transition from wakefulness to sleep or stay there if already asleep. For example, if you’ve ever gone to bed at a reasonable hour but found yourself just barely too warm for blankets and couldn’t get comfortable, your body could have been struggling to cool you down for bed.
This is because nesting behavior is also tied to the circadian rhythm and the first phase of rest, NREM according to a study conducted at the Imperial College of London. The study suggests that as our body cools, we seek out warmth and engage in nesting to prepare for rest. This causes our body to cool more in response to the warmth of blankets or sleeping partners, which guides us into NREM periods. When our body fails to cool in response to nesting behavior or the correct time for rest, you may experience insomnia.
However, this isn’t the only time of day when our bodies purposefully regulate temperature, throughout a twenty-four-hour cycle, body temperature will fluctuate a few degrees from the “normal” 98.6°F. Rising a few degrees into the afternoon, and then dropping a few degrees gradually until just before you wake up, thermoregulation can help manage alert and drowsy states, according to Swiss scientists, waking you up as temps rise, and making you sleepy as they drop.
Thermoregulation and the Sleep Stages
Thermoregulation isn’t just about getting to sleep, however. The Imperial College study found that the neurons in the brain that work on the onset of NREM periods are the same neurons that facilitate body cooling. This partially explains why these functions tend to happen at the same time, and sheds light on why REM and wakeful periods are accompanied by less regulation of brain temperature than NREM.
As we need both REM and NREM periods to function, the circadian rhythm plays an important function in creating the optimal temperature for each stage, improving the functionality of sleep and the way our bodies store and use energy.
To sync the circadian rhythm, some studies show warming the body before rest can improve the quality of slow-wave sleep and help maintain it in depressed patients whose temperature rhythms are out of sync.
Learn More: Why Heat Makes You Sleepy
Why Do Some People Sleep Hot?
While many people may say they sleep hot, that doesn’t always mean the same thing to different people. While some people are out there trying to fall asleep in a room with zero ventilation on the fifth floor and no air conditioning in the summer, wondering why they aren’t having success, others have more perplexing concerns. Not that both circumstances aren’t valid, some problems just have simpler explanations than others.
Some of us wake up in normal temp rooms with soaked sheets, panting from a hot flash, and others simply can’t bear the thought of blankets and spend hundreds of dollars in electricity because of the sheer volume of fans aiming at their bed all night.
Whether you’re a victim of hot flashes or you’re just a warm sleeper, there is probably an explanation for your struggle, and we aren’t about to tell you to suck it up. We’ve established that temperature is an important factor in improving sleep propensity, or the ability to transition into sleep. However, sometimes this mechanism seems to be more of a coincidence than a comfort feature supplied by the brain.
For example, while NREM onset is accompanied by a drop in brain temperature, brain temperature during REM is essentially determined by the room temperature according to a recent study in Current Biology.
This means that if your bedroom is abnormally hot while you’re deep in a REM period, it could interfere with your rest as your body may wake you up to cool you down. If you are suffering a loss of REM sleep because of going to bed late or drinking alcohol, you may find that you wake more frequently because you spend more time in REM periods, and therefore lose the heat regulation you might have had.
Additionally, disruption of the circadian rhythm can happen from a variety of factors, including sleep deprivation, jet lag, and living in perpetually dark locations, according to a University of Leeds study. These disruptions can add up and can interfere with your body knowing when to cool down, making it difficult for you to fall asleep. This means insomnia one night could make it harder to rest the next night, and when you do finally sleep, you may be hotter and not feel as rested.
Even if your circadian rhythm is functioning properly, a Harvard scientists suggest that those who are physically fit or who are used to warm climates sweat more efficiently, meaning you begin to sweat at a lower temperature than others. This could be good news if you need to stay cool while exercising but may turn into a nightmare if you frequently wake in a swamp.
For those who aren’t as fit, your body may not be as efficient at cooling you down even if your circadian rhythm is in check and trying to cool you at the right time.
A study at Pennsylvania State University suggests that wearing socks at night can make it harder to release the heat that is brought to the surface of the skin as the body aims to cool down. While the heat transfer may make us feel hotter at the skin level, it normally shouldn’t interfere with sleep unless there is a barrier stopping the heat from being released from the skin. Tight clothing, socks, poor air circulation, and humidity could contribute to sleeping hot even when your body is functioning properly.
Additionally, your body’s ability to cool is related to your overall body mass, diet, and endocrine levels, but probably not the climate you live in, according to the Handbook of Biological Effects of Electromagnetic Fields. So rather than assuming the Arizona sun or Canadian winter is the problem, you may want to look into more internal or behavioral factors that could be causing you to sleep warm.
…While Others Sleep Cold?
Just like for those who sleep warm, the loss of thermoregulation during REM periods could lead to feeling cold if your environment is colder than your body temperature. If you like to sleep with the window open without any blankets, you may wake up too cold during the REM cycle, especially if you are spending more time in REM than NREM due to alcohol consumption or other factors. The elderly may be at a higher risk for sleeping cold as they tend to have decreased vascular volume and less efficient heart function, both of which are used to regulate body temperature.
However, sleeping cold isn’t likely to wake you up as much as feeling warm as being cooler tends to be ideal for sleep, especially if you are bundled up enough. If you find yourself freezing while bundled up in blankets, it may be a good idea to check your temperature to ensure you do not have a fever or see a doctor who can determine other factors that could be affecting your ability to regulate body temperature.
In some cases, circulatory disorders can affect the body’s ability to thermoregulate through vasoconstriction or vasodilation. In other cases, feeling cold at night can occur because you recently ate a large meal. As it takes energy and heat to digest food, this may draw blood away from the surface of the skin. If your outer extremities are always cold when you go to bed, you may consider talking to your doctor about healthy ways to improve your circulation or try eating earlier in the day.
How Sleep Deprivation Affects Energy Efficiency
Sleep quality can be severely reduced due to skin temperature, rapid temperature change, and sweating. So thermoregulation isn’t just about comfort; if it’s not functioning properly you could experience sleep deprivation, which sadly has consequences that extend further than bags under the eyes and drowsiness.
The next day, sleep deprivation could decrease evaporative and dry heat loss during moderate-intensity exercise, according to the American Psychological Society. This means while you may feel more tired during exercise because you missed out on some muscle repair during sleep, you could also struggle to regulate temperature the next day, making you more tired faster, and potentially decreasing energy, stamina, and athletic performance.
Throughout the phases of NREM and REM, your body stores energy replenishes depleted hormones, boosts your immune system, and repairs the body. When you miss out on these vital functions, be it due to feeling hot, cold, or any number of factors, your cognitive function, energy stores, emotional processing, muscle growth, could all be affected.
Using Thermoregulation to Your Advantage for Better Sleep
The good news is there are many proven ways to sync up your thermoregulation and use it to your advantage to improve your quality of rest and your waking body function by default. A study published in Psychology and Behavior shows that you can improve your sleep propensity (or ability to transition from awake to asleep) at home by warming your skin to the level that normally occurs before you fall asleep.
This is because it simulates the response of vasodilation that warms the skin, causing the body to react by cooling you down via vasoconstriction, according to the study. While it may seem counter-intuitive if you’ve been feeling too warm, it works, and you can do it in a variety of ways.
Timing Your Exercise
Timing your exercise to a little while before bed can help increase your body temperature naturally so the body can respond with vasodilation to release body heat, and cool you down for bed. While we wouldn’t recommend exercising vigorously just before bed as that could keep you up longer, a brisk walk or jog about an hour before bed could help sync up your circadian temperature and help you get to sleep on time, according to recent research published by Frontiers in Neurology.
Keep in mind that if you have cardiovascular or heart issues, this may not be the best option for you as some studies suggest blood pressure can rise a few hours before you wake up, so elevating it may not be safe for everyone. If you have concerns, ask your doctor about your limits.
Rethink Your Bedtime Snacks
It takes a lot of energy to break down food, so if you give your body a big carb-heavy and fatty meal to work on just before hitting the sack, don’t be surprised if you start to warm up, or even or wake up with cold hands and feet because all the blood in your body is rushing to your stomach to aid digestion.
If your stomach is growling, good bedtime snacks are usually light, mostly protein, and easy to digest. Lactose intolerant people who love cheese, we are looking at you.
Adjust Bedroom Temp
During REM periods, body temperature is mostly based on external factors. To help ensure you don’t overheat while you dream, keeping your room cool (between 60°F and 72°F for adults) could provide some health benefits, help you fall asleep faster, and improve the quality of REM sleep.
Read More: Sleeping in a Cold Room
Breathable Pajamas
If you find yourself too hot, breathable cotton pajamas could help wick away sweat keeping you dry and cool, and are likely to not get in the way or your skin’s duty to release heat. If you’re extra brave, you could even try sleeping naked. It may be better for you anyway.
Taking a Hot Bath Before Bed
A study at Loughborough University shows that immersing yourself in hot or warm water before, (but not immediately before) going to bed can help you fall asleep faster, and help you sleep more deeply. This is known as the ‘Warm Bath Effect’ and it’s supported by a variety of studies.
However, before you get ahead of yourself, taking a warm shower is likely not as effective at raising core body temperature as there is less total body surface area exposed to the heat. Soaking in a hot bubble bath (bubbles add insulation) for about ten minutes should help raise the core body temperature to 100°F, where it should begin to cool naturally, creating the perfect drowsy environment for sleep.
Different Mattress and/or Bedding
Most modern mattresses have something to offer in the way of cooling bed mechanisms, but not all are created equal. Hybrid mattresses with springs tend to excel in air circulation, and copper-infused gel and convoluted foam can certainly help. If you already have a great mattress, bedding is the next on the list to consider.
View Our Guide: Best Cooling Mattresses
Breathable materials such as cotton can help promote airflow and wick away moisture, as well as preventing heat retention where you don’t want it. Before ditching all your cozy blankets, however, keep in mind that as you learn how to effectively help your body cool, you’ll probably want a warm and cozy bed to retire to.
Feeling uncomfortably hot is probably one of the most frustrating sleep-related concerns, but it doesn’t have to be the hardest to overcome. Sleeping hot has probably made it harder for you to fall asleep, stay asleep, and you may even feel less rested when you wake up. However, with a few new habits and some tools to help you stay cool, your body should be able to manage temperature on its own, helping to reduce temperature-related insomnia and improve your quality of rest.
Some people may struggle more than others, but we feel confident that this information will point you in the right direction to take the next step in improving your experience.
Jill Zwarensteyn
Editor
About Author
Jill Zwarensteyn is the Editor for Sleep Advisor and a Certified Sleep Science Coach. She is enthusiastic about providing helpful and engaging information on all things sleep and wellness.
Combination Sleeper