Why Do We Fall Asleep More Easily When It’s Cool?

5 min read

Last Modified 2 June 2021 First Added 20 September 2017

By Russell Foster

Many factors affect our sleep, including children, pets, noise and light pollution, bed partner, alcohol, sleep medications, exercise and, of course, temperature. Children, pets, snoring and traffic or aeroplane noise will wake us up, whilst alcohol and sleep medications sedate us. But why is it so difficult to sleep when it is hot? There is a strong relationship between sleep and body temperature.

The circadian clock

Our body clock, also called a circadian clock, drives a strong 24-hour rhythm in core body temperature. It increases a few hours before we wake, remains high during the day and, in anticipation of sleep, drops around 9pm to reach its low point around 4am. The low to high change is about 36.5 – 37.5 °C (97.7 – 99.5 °F). Not a huge variation, but a very important one for sleep regulation.

It seems that the evening drop in body temperature acts as an additional signal to help promote sleep and that the lowered temperature at night helps sustain sleep. But how do we know this? Studies in the laboratory have shown that wearing special cooling clothing or keeping people in environments that lowers skin temperature by just 1 °C can decrease the time it takes to get to sleep, reduce waking up in the middle-of-the-night and can even help you sleep longer.

Alarm clock on bed

This change in body temperature is driven by our circadian clock, and then fine-tuned by other factors. Our metabolic activities break-down food to fuel the processes of the body that keep us alive, and heat is produced as a by-product. By lowering the body’s metabolic rate in the evening, less heat is produced. If we get too hot, then our metabolic rate will increase and generate more heat. Normally, of course, the body can take corrective measures to stop this overheating.

Read more: How To Reset Your Body Clock [Infographic]

Heat loss

In addition to a lowered metabolism, a drop in body temperature is achieved by promoting heat loss from the skin. The blood vessels of the skin, and particularly the vessels in the hands and feet expand, a process called “vasodilation”. This vasodilation is partly controlled by the circadian clock and allows increased blood flow from the centre to the surface of the body in the evening, and the transfer of heat from the blood through the skin to the environment, which is usually cooler than the skin.

In addition, hairs on the skin will trap more warm air if they are standing up, and less if they are lying flat. When we are hot the tiny muscles in the skin relax, allowing the hairs to lie flat and increase heat loss.

Couple struggling to sleep due to heat

Why are hot nights so uncomfortable?

So, on a hot night there are multiple problems. The increased heat tends to raise our metabolic rate which generates more body heat and so increases body temperature further, whilst at the same time a hot environment slows heat loss from the skin. Both combine to reduce that all important sleep signal – the normal drop in body temperature. But there are added problems.

The vasodilation of the blood vessels to promote heat loss will reduce blood pressure, leading to a reduction in the levels of oxygen and nutrients supplying the needs of the body. This can cause fatigue. Fatigue is not tiredness, but a physical and mental weariness that can prevent sleep. Fatigued individuals may go to bed but fail to sleep. Added to this there is the problem of sweating which can make us feel even more uncomfortable and less able to sleep. Excessive sweating also leads to dehydration making us feel thirsty, irritable and fatigued, all of which prevent restful sleep.

Related: How To Beat Jet Lag And Sleep Well On Holiday

Staying cool

Other than installing air conditioning or wearing air conditioned pyjamas is there anything much we can do on those hot nights?

Fan in bedroom

  • A fan can help because it increases evaporative cooling from the skin surface, especially if combined with a water spray bottle for misting the skin.
  • Abandoning pyjamas and nightdresses will help evaporative cooling, but do try not to frighten housemates!
  • Body temperature rises after we eat so try not to eat too close to bedtime. Exercise will also raise core body temperature, so again try not to exercise close to bedtime.
  • A warm bath or shower 60-90 minutes before bedtime can be helpful as it will promote vasodilation and heat loss.
  • A soft mattress and pillows will envelop the body and insulate it from heat loss, so consider alternatives.
  • As heat rises consider sleeping in a lower floor room.
  • During the day keep the bedroom curtains closed to block out the heat from the sun.
  • If the outside temperature is warmer than inside close the windows, but ideally keep doors and windows open to allow air to circulate.
  • It is essential to stay hydrated and keep a water bottle by the bed.

If all else fails watch the box set of Sir David Attenborough’s “Frozen Planet”!

Related: The SMC Ultimate Guide To Staying Cool During Hot Nights

You can read more of Russell’s work in his books, VSI Sleep and VSI Circadian Rhythms.

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