What Is a Tropical Night, and What Does It Do to Sleep?

A tropical night is a night when the air temperature never drops below 20°C. Europe now tracks them as a climate warning sign, while Singapore has one almost every night of the year. Large sleep datasets link hotter nights to slightly shorter and lighter sleep, with older adults and people with higher body weight affected most, and there is little evidence that our sleep adapts quickly. One part you can control is the heat right around your body.

What counts as a tropical night

The definition is simple. The EU's Copernicus Climate Change Service describes a tropical night as one when the temperature does not fall below 20°C.

Core body temperature normally drifts down as you fall asleep, and warm, humid air reduces how much heat your skin can give away. That is why a warm night is about more than comfort.

Europe is counting more of them

Tropical nights are spreading across Europe:

• In 2024, Europe recorded its second highest number of tropical nights on record, behind 2010, according to the Copernicus European State of the Climate report.
• The share of Europe seeing at least one tropical night a year rose from around 20% in the 1970s to around 34% in the 2010s.
• In summer 2024, southeastern Europe had a record 23 tropical nights against an average of eight.
• In 2025, parts of the Iberian Peninsula and the Mediterranean coast saw up to 30 more tropical nights than average, and Vestfold, Norway, set a new record of 10 consecutive tropical nights.

A study of 75 weather series across Spain from 1970 to 2023 also found that tropical nights became more frequent and more intense, with longer runs of consecutive warm nights and high humidity in coastal areas during those episodes.

Sleep researchers are paying attention. In October 2026 the Journal of Sleep Research published a piece by researchers at the University of Rome Tor Vergata asking whether European heatwaves are an underrecognised methodological challenge for sleep research itself. The question shows how far night heat has moved into mainstream sleep science.

Singapore lives with them every night

By the European definition, Singapore rarely gets anything else. The Meteorological Service Singapore notes that night-time minimum temperatures usually do not fall below 23 to 25°C. Relative humidity often climbs above 90% just before sunrise, and the annual average is around 82%.

A 2026 Singapore study followed 18 adults aged 65 to 78 with smartwatches and wearable sensors and collected over 2,000 real-time surveys. Indoor night-time conditions often exceeded outdoor temperatures, even though participants had access to air conditioning. The researchers also noted that by 2050, one third of Singapore's population will be 65 or older.

What the sleep data shows

Each hot night costs only a little sleep on average. The effect is real, though, and it repeats.

| Study | Who and what | Main finding | |---|---|---| | Zhou et al., 2026, BMC Public Health | 7,682 people in China, about 1.25 million nights tracked with wrist wearables | In the hot season, each 1°C rise in maximum night temperature went with about 0.9 minutes less total sleep, mainly lighter sleep | | Wanchaitanawong et al., 2026, SLEEP abstract | 201,619 users, about 24.2 million nights, under-mattress sensors | Warmer and more humid nights shifted sleep toward lighter stages and slightly raised breathing disturbance scores | | Chevance et al., 2024, Sleep Medicine Reviews | Systematic review of real-world studies worldwide | Higher outdoor or indoor temperatures generally linked to worse sleep quality and quantity |

Two caveats. The China study used city-level outdoor temperature, not bedroom readings, and the under-mattress data is a conference abstract from a commercial device. Neither proves cause on its own, but together with the review they point the same way.

Who feels it most

The same groups show up again and again.

Older adults. In the China data, people aged 65 and over had the biggest heat-linked sleep losses. A global wristband study by Minor and colleagues, cited by the same team, found the per-degree impact in older adults was more than twice that in middle-aged adults. In Queensland, Australia, a study of 47 adults aged 65 and over tracked 14,179 night-time hours. Compared with bedrooms under 24°C, rooms at 28 to 32°C had 2.9 times the odds of a clinically relevant drop in heart rate variability, a marker linked to the body's overnight recovery.

People with higher body weight. Participants with a BMI of 28 or more also lost more sleep on hot nights, possibly because higher body weight makes heat harder to shed.

People with less control over their room. Sleep in lower income Chinese cities was linked to night temperature, while higher income cities showed no significant link. The authors think better indoor cooling explains much of that gap.

Do we get used to it?

Not quickly, according to the evidence so far. The 2024 systematic review found the effect of heat is stronger in the hottest months, in vulnerable groups and in the warmest regions, and reported limited evidence of fast sleep adaptation to heat.

What to do on a tropical night

None of these studies tested a specific fix, so treat these as practical steps rather than cures.

1. Cool the bed, not just the room. Most of the heat that matters is trapped between you and your bedding. We looked at this in our piece on bed microclimate versus room air.
2. Plan for the hours after the air conditioning switches off. If you use a timer, humidity and heat can creep back before dawn, as we covered in this look at humidity after the AC turns off.
3. Keep air moving. A fan helps more in some conditions than others; see what studies show about fans in humid heat.
4. Keep your routine steady. Do not try to force sleep on a bad night. The European Insomnia Network's heatwave guidance builds on elements of cognitive behavioural therapy for insomnia, with extra advice for vulnerable groups such as older adults, children and pregnant women.
5. Choose breathable bedding. If you like the settling feel of a weighted blanket but sleep hot, pick one built for warm nights. A cooling weighted blanket pairs gentle pressure with a cooler-feeling surface; you can see how one is made on our specifications page.

If poor sleep lasts for weeks regardless of the weather, speak with a doctor.

The takeaway

Europe has started counting tropical nights because they are becoming more common and they cost sleep. Singapore shows what life looks like when nearly every night is one. The loss per night is small, the effect is strongest in older adults and people who cannot cool their rooms, and there is little sign that we simply adapt. Managing the heat right around your body is a practical place to start.

Sources

1. Copernicus Climate Change Service. What is a tropical night? https://climate.copernicus.eu/what-tropical-night
2. Copernicus Climate Change Service. European State of the Climate 2024: Thermal stress. https://climate.copernicus.eu/esotc/2024/thermal-stress
3. Copernicus Climate Change Service. European State of the Climate 2025: Thermal stress. https://climate.copernicus.eu/esotc/2025/thermal-stress
4. Copernicus Climate Change Service. European State of the Climate 2025 report. https://climate.copernicus.eu/sites/default/files/custom-uploads/ESOTC-2025/Press%20Resources/European_State_of_the_Climate_2025-no_layout_version.pdf
5. Correa J, Dorta P, López-Díez A, Díaz-Pacheco J. Analysis of tropical nights in Spain (1970 to 2023): minimum temperatures as an indicator of climate change. International Journal of Climatology. 2024. https://doi.org/10.1002/joc.8510
6. Amicucci G, Centonze D, Liguori C. When Charon knocks at the sleep laboratory: are European heatwaves an underrecognized methodological challenge for sleep research? Journal of Sleep Research. 2026. https://doi.org/10.1111/jsr.70465
7. Meteorological Service Singapore. Climate of Singapore. https://www.weather.gov.sg/climate-climate-of-singapore/
8. You et al. Evaluating heat exposure and vulnerability among older adults using wearable technology to support aging in place. IOP Conference Series: Earth and Environmental Science. 2026. https://doi.org/10.1088/1755-1315/1582/1/012052
9. Zhou P, Shi Y, Liu J, et al. Nighttime temperature and sleep duration in the Chinese population. BMC Public Health. 2026. https://link.springer.com/article/10.1186/s12889-026-26308-5
10. Wanchaitanawong et al. Effects of heat and humidity on sleep parameters: insights from 24.2 million nights using an under-mattress sleep monitoring device. SLEEP (abstract supplement). 2026. https://doi.org/10.1093/sleep/zsag091.1269
11. Chevance G, Minor K, Vielma C, et al. A systematic review of ambient heat and sleep in a warming climate. Sleep Medicine Reviews. 2024;75:101915. https://doi.org/10.1016/j.smrv.2024.101915
12. Minor K, Bjerre-Nielsen A, Jonasdottir SS, Lehmann S, Obradovich N. Rising temperatures erode human sleep globally. One Earth. 2022. https://doi.org/10.1016/j.oneear.2022.04.008
13. O'Connor et al. Effect of nighttime bedroom temperature on heart rate variability in older adults: an observational study. BMC Medicine. 2025. https://doi.org/10.1186/s12916-025-04513-0
14. Altena E, Baglioni C, Sanz-Arigita E, Cajochen C, Riemann D. How to deal with sleep problems during heatwaves: practical recommendations from the European Insomnia Network. Journal of Sleep Research. 2023. https://doi.org/10.1111/jsr.13704