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.
Human beings are endotherms - able to thermoregulate - , that is, maintain their 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, which 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.
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.
Graph courtesy of National Center on Sleep Disorders Research
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.
In one study, subjects were exposed to a range of temperatures during sleep. Based on animal models, the researchers expected REM sleep to cause difficulty in thermoregulation, but the results showed that there was very little disruption of thermoregulation during REM and other sleep stages. The subjects shivered slightly in cold temperatures during sleep stages 1 and 2. Although skin temperature increased as the subjects were exposed to higher temperatures, their core temperature readings did not change.
A recent Dutch study shows just how important temperature is when it comes to sleep quality and fragmentation. Fitting human subjects with thermosuits, the scientists were able to raise skin temperature less than a degree Centrigrade without affecting core body temperature. The changes were dramatic. People didn't wake up as much during the night and the percentage of the sleep spent in stages 3 and 4 (deep sleep) increased. The effects were most pronounced in the elderly and in people who suffered from insomnia. A 0.4 C increase in skin temperature caused a decline in the probability of early morning waking from 0.58 to 0.04.
The same researchers found that people with narcolepsy tend to have higher skin temperature when asleep, and also when awake. They speculated that that hypocretin (orexin) deficiency in the brains of narcolepics affects skin-temperature regulation.
Other studies have shown different thermoregulatory responses of human subjects, depending on the sleep stage and temperature of the environment. Thermoregulation is less efficient during deep sleep than light sleepS. When subjected to different environmental temperatures, regulatory processes were affected. Scientisgts found an overly warm or cool bedroom temperature disturbed sleep. Time in REM sleep decreased, as did deep sleep to a lesser extent.
However, warmth beforehand improved sleep, especially slow-wave sleep time. In depressed patients, sleep is disturbed as well as body temperature rhythms. In these patients, a warm temperature before sleep might be helpful.
Here's an interesting fact: you don't sweat or shiver during REM sleep. Sleep researcher Jim Horne compares 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 Horne thinks it is possible that adults use brown fat to keep from cooling too much during REM.
Some scientific papers characterize REM as "poikilothermic sleep". Poikilotherms are organisms like frogs in which the body temperature varies. You don't see this term used often with regard to human sleep, but it is technically accurate.
You really can't change your body temperature much without getting severly ill. It is very dangerous if you temperature goes more than a few degrees above or below normal. However, many find that cooling down helps them get to 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. Recent research by Dutch scientists found that by increasing skin temperature the sleep quality in elderly people could be enhanced. People in the test wore heated thermosuits and with a slight (half a degree) increase in skin temperature were able to increase the length of time spent in slow wave sleep and decrease incidences of waking.
Related: beauty sleep