The Role of Temperature in​ Sleep

The thirteenth variable that I will be manipulating across a two week period to examine its impact on sleep is temperature. I will see if being too hot or too cold will have a substantial effect on sleep quality. 

Melbourne Just Sweated Through One Of Its Hottest Nights Ever Recorded

I will discuss what my data shows, how easy or difficult I found this strategy to implement, and what previous research says. These three factors will be combined for an overall score and grade on how effective modulating temperature is for improving sleep.

HOW COULD TEMPERATURE IMPAIR SLEEP QUALITY?

Living in Melbourne, Australia, there are usually a few nights every year which are unbearably hot. According to the Burea of Meteorology, 30.6 degrees Celsius is the hottest minimum temperature for a night ever in Melbourne, set in January 2017, with nights in the 27s and 28s occurring a few times in 2017 and 2018 too.

For people that are fortunate to live in places with excellent air conditioning, this may not be a problem. For many others, however, they can lead to a very long night, with frequent tossing and turning. I even remember when I was younger sometimes having a cold shower or jumping into a cold pool late at night with the hope of being able to then fall asleep once I went back to bed.

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The opposite can also be true, with the minimum temperature overnight reaching as low as minus 2.8 degrees Celsius in July. Not quite as cold as North America or Europe, but still cold enough to make it difficult to go to sleep and stay asleep during the night.

THE EXPERIMENT

For the first week, I slept with the window open with only a sheet and a light blanket covering me. As it was the middle of Winter (July 2017), there were some cold nights, with the 2nd of July getting down to 0.8 degrees Celsius, but the 5th of July only dropped to 10.1 degrees.

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For the second week, I closed the window and went to bed with 2 blankets, socks on my feet, a beanie and a big hoodie. The temperature minimum outside varied from 3.2 to 9.9 degrees Celsius, but it felt more like 30 degrees to me with how many layers I had on.

Let’s see if being too hot or too cold was worse for my sleep…

THE OUTCOME

sleep diary temperature

Comparison: Too Hot vs Too Cold

Based on my sleep diary data, the findings were as follows:

  • The number of awakenings:
    1. Too hot – 0.86 per night
    2. Too cold – 1.14 per night
      • less is better
  • Time in bed:
    1. Too cold – 7 hours 42 minutes
    2. Too hot – 7 hours 32 minutes
      • 8 hours is ideal for me
  • Time to bed:
    1. Too cold – 12:04 am
    2. Too hot – 12:07 am
      • 11:30pm is ideal for me
  • Total sleep time:
    1. Too cold – 7 hours 11 minutes
    2. Too hot – 7 hours 8 minutes
      • 7 hours 30 minutes is ideal for me
  • Sleep onset latency:
    1. Too hot – 12.14 minutes
    2. Too cold – 14.43 minutes
      • quicker is better
  • Wake after sleep onset:
    1. Too hot – 12.86 minutes
    2. Too cold – 14.29 minutes
      • less is better
  • Rise time:
    1. Too hot – 7:40 am
    2. Too cold – 7:41 am
      • 7:30am is ideal for me
  • Sleep quality:
    1. Too hot – 4.57/5
    2. Too cold – 4.29/5
      • higher is better
  • Sleep efficiency:
    1. Too hot – 94.48%
    2. Too cold – 93.29%
      • higher is better

AND THE WINNER IS…

With a count of 6 points to 3 points, being too hot is better for sleep than being too cold, with less time taken to fall asleep, fewer awakenings during the night, less time awake during the night, and better subjective sleep efficiency and sleep quality.

IS MODULATING TEMPERATURE A GOOD SLEEP STRATEGY?

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IS IT EFFECTIVE?

Maybe. If you look at the differences between the two on their outcomes, the differences are actually entirely negligible. I slept pretty well both when I was too cold and when I was too hot. Not knowing how temperature would affect my sleep, I tried to compare the two extremes. It is possible that if I was Goldilocks and slept at the exact right temperature that my sleep could have been even better, but I’m pretty happy with how it was on both weeks anyway. Chances are that temperature isn’t the most critical factor for a good night’s sleep.

I, therefore, give the effectiveness of this strategy a 12/25.

CAN IT BE APPLIED?

Yes. It is definitely possible to open windows and put on more layers for most people, especially in Australia. It would be harder for rough sleepers to get warm during the winter without a blanket or extra clothes. It would also be harder for people to cool down without airconditioning on a hot summer’s night.

I, therefore, give the applicability of this strategy a 15/25. 

IS IT SCIENTIFIC?

Haskell and colleagues (1981) assessed six male colleagues who slept in just shorts on top of a bed with at various temperatures ranging from 21 degrees Celsius to 37 degrees Celsius. The 21-degree condition was the most disruptive to sleep, with the colder temperatures generally being more disruptive to sleep than the warmer temperatures, based on the polysomnography findings (Haskell et al., 1981). Some individuals appeared to be much more sensitive to temperature than others, although again there could have been other factors impacting their sleep than just temperature.

Another more recent study by Lan and colleagues (2004) suggested four main findings from their research into the role of temperature on sleep:

  1. Sleep quality is sensitive to changes in air temperature for humans.
  2. Skin temperature increases or decreases concurrently with changes in air temperature during sleep.
  3. When temperatures moderately deviate from what feels comfortable to someone, they take longer to fall asleep, experience less deep sleep as recorded by EEG, and report poorer sleep quality the next morning.
  4. People should try to sleep at the room temperature that feels comfortable to them.

I, therefore, give the science of this strategy a 30/50.

Overall, modulating temperature as a way to sleep better gets a score of 12/25 + 15/25 + 30/50 =

57/100: Pass

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WHAT I RECOMMEND

 

If you worry that you are too hot or too cold when going to bed at night, try to find some cheap and easy strategies that get you to a point where you feel comfortable. While 18 – 24 degrees Celsius is sometimes recommended for people to get the best night’s sleep, try to focus on how you feel and not what the numbers say.

If you wake up shivering or sweating and you believe this is what woke you up, the temperature could be playing a role in reducing your quality of sleep. Outside of this and the few other extreme cases, there are probably many more strategies that you could if you want to improve the quality of your sleep.

I have heard people talk about the Chilipad, which helps you to heat up or cool down your side of the mattress to the temperature that you want it to be, but at over $1000 for a king mattress, I’d recommend spending your money elsewhere.

Thanks for reading! If you would like a personalised sleep report and the five things that you could do to best improve your sleep, please check out our services.

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