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Too hot to sleep? Here’s why

Bushfires are quite appropriately dominating our nation’s concerns during the current Australian heatwave. But for many, the struggle to sleep through soaring temperatures is a personal inferno that dominates…

Environmental heat disturbs the delicate balance between sleep and body temperature. Steven Mileham

Bushfires are quite appropriately dominating our nation’s concerns during the current Australian heatwave. But for many, the struggle to sleep through soaring temperatures is a personal inferno that dominates conversation around offices and homes across the country.

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. But temperature changes become more complex during sleep as our temperature self-regulation varies according to sleep stage.

Research has shown how environmental heat can disturb this delicate balance between sleep and body temperature. An ambient temperature of 22˚ or 23˚ Celsius is ideal. Any major variation in this leads to disturbance of sleep with reduced slow wave sleep (a stage of sleep where the brain’s electrical wave activity slows and the brain “rests”), and also results in less dreaming sleep (rapid eye movement or REM sleep).

Indeed during REM sleep, our ability to regulate body temperature is impaired so in a clever sort of way the body “avoids” this stage of sleep during extreme cold or heat. A heat wave may cause several nights of fragmented sleep with less slow wave and REM sleep. This will certainly cause a correct perception of bad, restless sleep with consequent negative effects on mood and alertness.

In theory, it may also have subtle effects such as problems with complex memory retention, higher judgement (poorer decision making and increased risk-taking behaviour), blood pressure control and regulation of glucose in the body. The clear message is this: if you’re going to make some big decisions during a heatwave, sleep in a carefully controlled air-conditioned environment.

But apart from air-conditioning, what can you do to sleep better during a heatwave? Sleeping in the lateral position (on your side) with less contact with the mattress may be good but the body tends to do this anyway during sleep, in response to rising temperatures.

Cooling the central body with a wet cloth or towel makes sense. A cool shower may also help. It is important to avoid doing anything too strenuous in the hours before bed-time as this will make it harder for the body temperature to fall during sleep. And when you wake up hot, sticky and irritated because you don’t have air-conditioning or believe such devices are environmentally unsound, remember those fighting bushfires - it could be a lot worse.

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6 Comments sorted by

  1. Colin MacGillivray

    Architect, retired, Sarawak

    Clearly different temperatures for different folks. My preference is 28C with 90% humidity for a good night's sleep. (My home is 160kms north of the equator).
    If one's skin temperature is around 32C why is 10 degrees lower the "ideal" for sleeping? Primitive humankind didn't have aircon but evolved near the equator.
    Certainly over 30C is getting warm but 22-23 seems cool.

    1. George Takacs


      In reply to Colin MacGillivray


      You want the surrounding temperature to be lower than your surface temperature so that the rate at which you lose heat to your surrounds is approximately the same as your metabolic rate.

    2. Sue Ieraci

      Public hospital clinician

      In reply to Colin MacGillivray

      It seems likely that long-term adaptation must occur - both Innuit in the Arctic circle and people living on the equator gotta sleep.

  2. Comment removed by moderator.

  3. Peter Hewson


    Ron, an interesting explanation. I do already follow your recommendations although sleep and I are often not bed pals. Being late middle aged the need to pee during the night is a pain but equally I have found lack of hydration an issue.

    Last night (in Sydney 30+C at a late bed time) I did run the air-con (and some fans to circulate air into the bedroom). What surprised me was that even with all the noise I slept well in the relative coolness.

    I have a sleep clinic as a next door enighbour. In winter the ruddy place keeps me awake with their air-con units switching on and off!