With summer on the horizon and temperatures rising in Belgium, sleepless nights are becoming more frequent for many of us here in Brussels.
A summer sleep drought can have a serious effect on our bodies. With early sunrises and late sunsets, we get less of the darkness we need to make the sleep hormone, melatonin. This might explain the reason it can be hard to drop off after a day out in the sun, according to a German study published in 2023.
Discomfort during heat is quite common, say sleep researchers. Heat fundamentally interferes with the biological mechanisms that allow humans to sleep properly, with consequences that can extend far beyond simple fatigue.
"The human body's core temperature is around 36.5°C, but it naturally drops in the evening. This decrease is associated with the production of melatonin, the hormone that facilitates sleep," Médhi Gilson, neuropsychologist, sleep researcher and somnologist at the Université Libre de Bruxelles (ULB) told The Brussels Times.
REM sleep
That nightly cooling process is essential. It effectively "opens the sleep window", helping the body drift into restorative sleep. But during heatwaves, especially when temperatures remain high overnight, that delicate mechanism begins to fail.
"When the bedroom is too hot, this process is disrupted," Gilson said. "It affects falling asleep, but also access to deep sleep and REM [rapid eye movement] sleep." REM sleep is the most "active" period during which we dream.
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A review published by the European Insomnia Network noted that heatwaves are becoming more frequent across Europe and that night-time temperatures are rising particularly quickly.
The paper explains that environmental temperatures above the body's thermal comfort zone suppress the normal decrease in core body temperature at sleep onset, leading to greater wakefulness and reduced sleep. Deep sleep is especially important for physical recovery. REM sleep, meanwhile, plays a major role in emotional regulation and mental health.
"We think REM sleep participates in mood regulation because the emotional centres of the brain are particularly active during this phase," Gilson explained. "Lack of REM sleep can disrupt emotional regulation processes and favour anxiety or depressive symptoms."
'Sleep debt'
According to Gilson, the effects are especially visible during prolonged heatwaves, when several consecutive nights of poor sleep accumulate into what experts dub "sleep debt".
"If sleep is disrupted over several nights, then yes, people can accumulate sleep debt," Gilson explained. "That can have consequences for both physical and mental health". The most common symptoms reported by Gilson's patients during hot periods include fatigue, irritability, nervousness and concentration difficulties.
"People are probably more irritable during extreme heat," Gilson said. "That is something I clearly observe in my practice".
Heat may also worsen psychological distress independently of sleep deprivation. "Beyond the sleep issue, heat during the day also has an important influence on mental health," Gilson said. "That is well documented scientifically."
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Effects on the elderly and young children
The elderly appear particularly vulnerable because their body's ability to regulate temperature decreases with age. Children could also be disproportionately affected. "It is not yet fully understood scientifically, but we strongly suspect children are more affected by tropical nights," Gilson explained.
The European Sleep Research Society (ESRP) warns that children exposed to excessive heat at night may experience altered thermoregulation, higher heart rates and disrupted sleep cycles. Experts fear that climate change could make these problems increasingly common in cities such as Brussels, where buildings often retain heat long after sunset.
While Gilson notes he is not an urban planning specialist, he believes Brussels architecture may exacerbate problems. "I have the impression Brussels was originally built to conserve heat during winter – maybe not necessarily to deal with very high summer temperatures."
That urban "heat island" effect can make nights especially difficult in densely built neighbourhoods with limited vegetation and poor ventilation.
So how can people protect their sleep during heatwaves?
According to ESRP, the first priority is cooling the body gradually before bedtime. It recommends keeping bedrooms around 19°C when possible, or at least below 25°C.
Gilson also advises taking a lukewarm shower before bed to help the body release heat more effectively. Cold showers may, paradoxically, stimulate the body and make sleep harder. Hydration is also paramount. "You should drink plenty of water during the day and keep a bottle near the bed," he says.
Ventilators can also provide relief by improving sweat evaporation and creating a sensation of freshness. ESRP note that fans consume far less energy than air conditioning, although caution may be needed for vulnerable individuals with chronic illnesses.
ERSP also recommend avoiding intense evening exercise, limiting alcohol consumption and maintaining regular sleep schedules despite difficult nights.
Alcohol
A cold beer on a summer night may be nice but the European Insomnia Network clearly state that "alcohol disrupts deep sleep continuity, increases dehydration and intensifies nocturnal sweating."
Perhaps most importantly, specialists say people should not underestimate the cumulative impact of repeated poor sleep during increasingly frequent heatwaves. Climate change is not only warming the planet during the day. It is quietly transforming the night as well.
Additional tips for better sleep
For the elderly
- Use ventilators carefully to improve airflow and sweating evaporation
- Avoid overcooling rooms below 17°C, which can also disrupt sleep
- Take a cool or lukewarm shower before bed
- Stay hydrated throughout the day
- Keep rooms dark and cool during both day and night
- Ventilate the bedroom during cooler periods of the day
- Wear light cotton clothing
- Maintain a regular sleep schedule
- Avoid intense physical activity late in the evening
- Use early morning hours for light activity when temperatures are cooler
For children
- Ensure children get the recommended amount of sleep for their age
- Keep a regular sleep-wake rhythm despite summer holidays
- Set a late bedtime even during hot evenings
- Encourage hydration throughout the day
- Avoid sugary snacks before bedtime
- Allow only light evening snacks if children go to bed later
- Encourage outdoor activity during cooler hours
- Allow naps, but not too late in the afternoon
- Promote calm bedtime activities such as reading, music or storytelling
- Keep children's rooms cool, ventilated and dark
The paper also includes recommended sleep durations:
- Newborns: 14–17 hours
- Infants: 12–15 hours
- Toddlers: 11–14 hours
- Pre-schoolers: 10–13 hours
- School-aged children: 9–11 hours
- Adolescents: 8–10 hours
For women and pregnant women
The ESPR highlights that women already experience higher rates of insomnia and poor sleep, and that pregnancy and postpartum periods are particularly sensitive to heat exposure.
- Pay particular attention to sleep quality during pregnancy and postpartum periods
- Follow all general heatwave sleep recommendations
- Use cooler early morning hours for light physical activity
- Reserve the hottest hours of the day for rest and relaxing activities
- Maintain hydration
- If possible, spend time in cooler environments during heatwaves
- Adapt routines individually depending on support systems and personal wellbeing
- Reduce physical and emotional stress during extreme heat
