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Even if you don’t tend to sleep hot or cold, you probably have some idea that your body temperature changes throughout the night. Body temperature correlates with overall sleep quality. It’s important to understand how thermoregulation works, so you can achieve a better night’s sleep.
Human beings are endotherms, which means we are able to thermoregulate, or maintain our body temperature. Body temperature is regulated through a balance of heat absorption, production and loss. Human temperature must be maintained within a fairly small range, up or down from the resting temperature of 98.6. Temperatures above 104.9 degrees Fahrenheit or below 92.3 degrees generally cause injury or death.
Humans have two zones to regulate, their core temperature and their shell temperature. The temperature of the abdominal, thoracic, and cranial cavities, which contain the vital organs, is called the core temperature. Core temperature is regulated by the brain. The shell temperature includes the temperature of the skin, subcutaneous tissues, and muscles, and it is more affected by external temperature. The core is able to conserve or release heat through the shell.
When the core temperature is too high, blood vessels in the skin dilate and heat is lost through their walls. (This is hardly news to observers; in Ancient Greece Hippocrates speculated that sleeping bodies feel cool to the touch because blood flows away from the skin) Sweat is also produced; it evaporates and lowers temperature. If a human is too cold, the blood vessels constrict, conserving heat. Blood is preferentially shunted to the internal organs and away from the skin and peripheral structures like limbs.
When you wake up, your body temperature is at its baseline of 98.6 degrees. Over the course of the morning through the late afternoon, your hypothalamus works to drive that up to 100.4 degrees. This rise in body temperature gives you energy, helping you stay alert. This partly why working out is so energizing – the rise in body heat makes you feel awake.
The 2 pm slump isn’t just an excuse dreamed up by cubicle workers – it’s a real phenomenon. In the mid-afternoon, your body starts to lower your body temperature to prepare you for sleep. At 5 am, a few hours before waking up, you’re at your lowest body temperature (96.4 degrees).
|Time||Body Temperature (degrees Fahrenheit)|
The hypothalamus regulates body temperature between 96.8 and 100.4 degrees Fahrenheit over each 24 hour cycle. During the normal human circadian rhythm, sleep occurs when the core temperature is dropping. Sleep usually begins when the rate of temperature change and body heat loss is maximal. The average adult’s lowest temperature is at about 5 AM, or two hours before waking time.
A cooler core body temperature is associated with sleep. Conversely, a warmer core temperature is energizing. Think about how awake you feel during exercising, and it starts to make sense. Human performance scientists have found a higher internal body temperature correlates with more alertness, better memory, and improved reaction times.
In relation to sleep cycle, early birds experience an earlier body temperature peak than night owls do. In the chart above, one might imagine their curve being shifted slightly earlier.
From your peak in body temperature in the early evening to the lowest point just before waking up, you experience a decrease in core body temperature of 2 degrees Fahrenheit.
The temperature of both the brain and the body fall during NREM sleep. The longer the NREM-sleep episode, the more the temperature falls. By contrast, brain temperature increases during REM sleep. The control of body and brain temperature is closely tied to sleep regulation.
Many mammals lose significant thermal regulatory capacity during sleep. Some animals like squirrels go into a torpor state during sleep, in which their body temperature dips well below the normal level for hours at a time. However, most research to date seems to indicate that humans do not have significant difficulty thermoregulating during sleep.
A recent Dutch study shows just how important temperature is when it comes to sleep quality and fragmentation. The researchers fit human participants with thermosuits. Raising their skin temperature less than a degree Centigrade resulted in dramatic changes in sleep quality. People didn’t wake up as much during the night and the percentage of the sleep spent in deep sleep increased. The effects were most pronounced in the elderly and in people who suffered from insomnia.
The same researchers found that people with narcolepsy tend to have higher skin temperature when asleep, and also when awake. This warmer skin temperature may help explain why they’re so prone to fall asleep.
Thermoregulation is less efficient during deep sleep than light sleep. This is why having a too warm or too cold bedroom temperature can affect your sleep and cause you to wake up during the night.
However, some warmth before bed can be beneficial to inducing sleep. Why does a warm (but not hot) bath help so many get to sleep? Because it ends up cooling you down, especially as you dry off and the residual water on your skin evaporates.
Here’s an interesting fact: you don’t sweat or shiver during REM sleep.
Sleep researchers have compared the REM non-thermal regulation period to that of normal functioning of babies, who neither sweat nor shiver even when awake. Babies control their body temperature, when it gets too cold, not by shivering but by use of so-called “brown fat” which is a type of adipose tissue well suited to generating heat. Adults have substantially less brown fat, adjusting for body weight, than babies do, but it is possible that adults use brown fat to keep from cooling too much during REM.
You really can’t change your body temperature much without getting severely ill. It is very dangerous if you temperature goes more than a few degrees above or below normal.
However, many find that cooling down before bed helps them get to sleep.
During REM sleep, your brain stops working to regulate your body temperature. Even the hypothalamus needs a break. This is why it’s so important that the external factors in your sleep environment (your room temperature, clothing, and bedding) are conducive to keep you cool.
The ideal temperature for falling asleep is in the mid-60 degrees Fahrenheit – between 60 and 67 degrees. Find the temperature that works best for you. It should be cool enough to help you fall asleep without waking up a few hours later shivering, without being too warm to cause you to wake up from sweating.
More tips to help you sleep cooler?
As mentioned above, a warm bath helps cool down your body temperature, as the moisture quickly evaporates from your skin upon stepping out of the tub. Try taking a bath 1 hour before bed for optimal effect.
Do you sleep hot? Besides being uncomfortable, excessive body heat while you sleep leads to less restful sleep and may cause you to wake up during the night, interrupting your sleep cycles.
The design of certain mattresses only makes matters worse. Different mattress types trap heat more than others – all you need to know is what to watch out for.
Firmness: the softer the mattress, the more it retains heat. A softer mattress will contour more to your body and cause you to sink deeper into the mattress. The mattress envelops your body and traps heat.
Your body weight: The more you weigh, the likelier you are to sink further into the mattress, causing it to envelop your body and trap heat. Not only does exercise help expend energy during the day, but it reduces your body weight, both of which will lead to more restful, cooler sleep at night.
Mattress type: Mattresses that contain foam are designed to conform to your body, so they’re the worst culprits for trapping heat. The worst mattresses for hot sleepers are hybrid, memory foam, and latex hybrid or latex mattresses. Look for mattresses that rate poor on contour ability but high on temperature regulation, such as waterbeds, innerspring mattresses, or airbeds. These mattresses do a better job keeping your body lying above the bed.
|Best Mattresses for Hot Sleepers||Worst Mattresses for Hot Sleepers|
|Water, Futon, Innerspring, Air||Hybrid, Memory Foam, Latex-hybrid, Latex|
One marketing ploy to watch out for: Many mattress retailers emphasize gel as a “cooling factor” in mattresses and pillows. In most cases, sleepers report the gel had no discernable difference on the bed temperature. Oftentimes, the gel is buried under multiple layers of foam, which reduces its effectiveness.
If you find yourself overheating while sleeping, try the following:
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