Extreme heat events are becoming more common, and can affect your health in a number of ways.Spencer Colby/The Canadian Press
The Sleep Whisperer is a series of columns offering insights and advice about sleep health. It is not a substitute for seeking professional medical care.
Heat waves are breaking temperature records in several parts of Canada. As extreme heat events are becoming more common in the context of climate change, you may be wondering how heat affects your health and how to best adapt. What may come as a surprise is that temperature spikes actually have more impacts on our physiology during sleep than when we are awake. Understanding the relationship between heat and sleep may therefore be key to staying healthy in a warming world.
Warmer temperature can be bad, but here is why acute heat waves are worse
Our body has evolved with some capacity to acclimatize its own temperature in response to progressive changes across seasons, but this process takes time. However, when we face sudden increases in ambient heat, the story is quite different, as our body triggers emergency physiological changes. For instance, blood flow rushes towards the skin in an attempt to transfer some of the heat out of the body. To achieve this, our heart needs to rapidly switch gears, working harder and faster. This not only increases cardiovascular risks, but also unbalances the autonomic nervous system in a way that may be less conducive to sleep. Furthermore, sudden temperature peaks can confuse the biological clock, which expects cooling temperatures at night to set the times at which our body is ready to fall asleep and wake up. When high humidity is part of the picture, it makes things even worse as our skin struggles to dissipate our internal body’s heat through evaporating sweat.
Sleep’s response to heat
All of this has measurable impacts on our sleep. A recent study linking meteorological data to sleep-tracker data in over 300,000 people around the world found that temperatures around 27 Celsius were linked to shorter sleep compared to more temperate weather. In fact, laboratory studies simulating heat waves showed a 14-minute reduction in sleep duration for every 0.1 C increase in core body temperature during the night.
The quality of sleep also melts down when temperatures rise. Research consistently shows that acute periods of warmer weather cut the time we spend in deep sleep and REM sleep. This is especially unfortunate since these sleep stages are critical periods when our body regulates its own temperature. If you have sleep apnea, warm nights may also worsen your breathing.
Since sleep plays a critical role in many dimensions of health, sleep difficulties triggered by heat waves worsen our susceptibility to other heat-related illnesses. Indeed, research is starting to map associations between how we sleep and the surge of mental health, brain and heart problems during heat waves.
Is it time for a ‘sleep divorce’?
How to protect your sleep health during extreme heat
With extreme heat warnings in place across many parts of Canada, here are a few tips to shield your sleep against the heat:
- Air your bedroom, keep your blinds closed and aim for temperatures below 25 C or, ideally, 19 C, if you can.
- Take a cool or lukewarm shower or foot bath before bed.
- Stay hydrated during the day; this will help your body regulate its temperature more efficiently during the night.
- Strip down for sleep. The more skin you expose, the more your body will expel heat and humidity. If you do wear pyjamas, opt for cotton fabrics, which are lightweight and breathable.
- If you do need to nap, keep it short (e.g. maximum 20 minutes) and before mid-afternoon.
- Watch your alcohol consumption. Alcoholic drinks may dehydrate you and worsen the effects of heat waves on sleep.
- Don’t fall into the trap of going to bed earlier after a bad night, as this can further reduce your deep sleep debt related to the heat wave.
- Embrace the mornings. You may wake up a bit earlier than usual because of the heat. If you feel that you won’t fall back asleep, get out of bed and start your day rather than lingering in bed. If this creates opportunities to do a bit of light exercise in the morning before the heats gets worse, this may boost your energy levels during the day and protect your sleep.
As we can increasingly forecast extreme heat events, there are also things you may be able to set in place before the next heat wave. To mimic natural acclimatization and slowly built heat tolerance, you can build gradual exposure to heat over time in the one to two weeks before heat strikes. This could be done by introducing 1-1.30 hours of exercise in progressively warm conditions, or taking warm baths. These techniques have been shown to enhance deep sleep later on when environmental temperatures rise. If you try this, make sure to stay well hydrated during those transitional periods.
Of note, both cooling with AC and ventilation with electric fans can improve sleep. You can do your part to slow down global warming by using ventilation when sufficient and leaving AC for the more extreme cases. Also, keep in mind that using AC all summer long may reduce your natural physiological adaptation to heat, which subjects your body to more abrupt temperature shifts when heat waves emerge.
Dr. Rébecca Robillard, PhD, is a clinical neuropsychologist and associate professor at the School of Psychology at the University of Ottawa. She also leads clinical sleep research at the Royal mental health hospital. She co-chairs the Canadian Sleep Research Consortium, a national hub of sleep scientists and clinicians.
