Humans have evolved to keep their bodies warm by shivering, burning fat, or using the brain to develop methods to adjust ambient temperature, for example, by manipulating fire.

While reading about how birds can withstand 15 degree drops in body temperature when it’s cold, and how alligators dangle in frozen water, I wondered what kind of temperature cycling humans might experience. I learned that our core body temperature changes by about 2 degrees daily, going down when we sleep and going up when we’re active. In fact, if we want to sleep we have to help it go down, and if we want to be active or think well, we have to help our body temperature go up.

Too hot to sleep? Here’s why, The Conversation, January 2013

Sleep and body control of temperature (thermoregulation) are intimately connected. Core body temperature follows a 24-hour cycle linked with the sleep-wake rhythm. Body temperature decreases during the night-time sleep phase and rises during the wake phase. Sleep is most likely to occur when core temperature decreases, and much less likely to occur during the rises.

Our hands and feet play a key role in facilitating sleep as they permit the heated blood from the central body to lose heat to the environment through the skin surface. The sleep hormone melatonin plays an important part of the complex loss of heat through the peripheral parts of the body.

At sleep onset, core body temperature falls but peripheral skin temperature rises.

Below is an article on body temperature in relation to sleep. I posted some excerpts.

Thermoregulation, Tuck, 8 January 2019

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 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. … 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).

Here’s a chart I found on Wikipedia that shows normal body temperature cycling. It doesn’t match the above numbers exactly, but even the article said there is variation. Also:

In relation to sleep cycle, early birds experience an earlier body temperature peak than night owls do. In the chart [below], one might imagine their curve being shifted slightly earlier.

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.

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.

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.

Thermoregulation in Humans: How Does Body Temperature Affect Sleep Quality?

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.

One kernel I walk away with, besides that we only have a narrow range of 2 degrees with which to play, is that sleep happens when the core is cool and the peripherals (hands and feet) are warm. If your core is warm (controlled by the brain) or your hands and feet are cold (controlled by environment and the brain) you’re going to have a hard time sleeping.