Heat is one of the biggest enemies of good sleep. This is why insomnia tends to get worse for many people during hot summer nights. The human body doesn't sleep well when it is hot. 

In this post, we dive deep into the science of sleep temperature. Why the heck does the body need a cool environment to sleep and what does heat do to sleep quality? 

How Temperature Affects Sleep 

Sleep, in humans as well as many other animals, is strongly linked to thermoregulation. That is, how the body maintains the ideal internal temperature. Thermoregulation is, in turn, controlled by the circadian rhythm, the 24-hour sleep and wake cycle we go through each day. 

The circadian rhythm is divided into two phases. During the sleep cycle, core body temperature begins to drop, heart rate and blood pressure reduces and we get sleepy. The wake cycle is characterized by an increase in core body temperature, higher alertness, and increased heart rate and blood pressure.  

Sleep is not what causes your body temperature to fall. Rather, the drop in body temperature is a precursor to sleep. Being too hot or too cold affects your ability to fall asleep, which is why you are more likely to get insomnia when it’s hot or chilly at night. 

Sleeping hot or cold also affects your sleep cycles. Researchers have found that high temperatures reduce both REM and slow wave sleep, the two main types of sleep. So not only does being too hot cut down on your sleep time, it also prevents deep sleep, which you need to wake up rested and refreshed.  

Sleep deprivation from reduced sleep duration and quality has far reaching effects on your health and productivity. It increases risk of diseases like hypertension and Alzheimers, worsens mental health, reduces concentration, lowers energy levels and affects your moods.   

Why Your Body Needs to Cool Down for Sleep

A lower core body temperature tells your body that it’s time to sleep. It is an important precursor to falling asleep and getting deep sleep. 

The circadian rhythm relies on several signals to run various processes in the body, including thermoregulation. The most important signal is light. As darkness approaches, our eyes detect less light and the internal circadian clock sends a signal to the hypothalamus in the brain to begin cooling your body. 

The reason you feel a bit colder at night is not just because there’s no sunlight, it’s also because your body reduces core temperature by a degree or two. 

Staying cool at night makes it easier for your body to achieve the ideal temperature for sleep, which helps you fall asleep quicker and stay asleep. 

Okay, so lower temperatures help kickstart our nightly journey to the dream world. But why exactly do we need to be colder to sleep?

That’s where researchers are not sure. There are several theories around. Some experts think that it allows certain processes in the liver, kidney and other organs to run better. Maybe it’s important for healing and recovery, which happens when we are asleep. 

Other experts say it is an evolutionary trait. Nights are cold and it would use a lot of energy to keep the body warm all night. So as a way to reduce energy needs, we have evolved to have lower metabolism at night, which lowers how much energy we need and generates less heat. Thus, the body gets colder at night. 

Easy Ways to Stay Cool When It’s Hot

Sleeping cool is easier said than done during hot summer nights. 

Chilly nights are easier to deal with. Simply cover yourself with a second or an extra-thick duvet, or wear warmer PJs. Dealing with heat is another matter altogether. 

The most important part of sleeping cool is your bedding. Get cool and breathable sheets like the Hercleon Jax sheets, use a lightweight comforter and sleep on a mattress that doesn't retain heat. 

Generally, bedding made from natural materials like linen, wool, silk, feathers, down, cotton, bamboo and Tencel are better at keeping you cool. They also wick away sweat. 

If it’s really hot at night, consider getting a cooling mattress pad or a bed cooling system such as BedJet or Sleepme’s Chilipad. 

Cooling your bedroom using central AC, a portable AC or a window unit will also help you sleep cooler. And if that’s not an option, turn on a fan and crack open a window to let the heat escape.