A good night’s sleep plays a crucial role in keeping the human body healthy. Healthy sleep at night is as important and essential for our well-being as a balanced diet and regular exercise, and without the former, the latter two might be ineffective. The long-term effects of repeated sleep deprivation include increased risks of hypertension, stroke, depression, diabetes, obesity, and heart attack.

But a worrying trend is creeping its way into our nights: we’ve been losing several precious minutes of our nighttime sleep to climate change. The implications are significant: a study, published this month, now claims that increasingly hotter nights may also lead to a rise in mortality rates due to the corresponding compromised sleep.

The study, co-authored by a group of scientists from South Korea, China, the United States, Japan, and Germany, looks at mortality data from 28 cities across China, Japan, and South Korea between the years 1981 and 2010. Along with this, they analyzed a value called Hot Night Excess (HNE), derived by calculating the excess sum of temperatures at night in these cities. In other words, the researchers looked at weather data from the same period as the mortality figures to find nights that were hotter than usual, took out the excess value from the temperatures of those nights, and calculated a sum of those temperatures.

On running a regression model using mortality data with HNE figures, the researchers found that HNE was significantly associated with higher death rates. For instance, the relative mortality risk was found to be 50% higher on days that recorded hot nights as opposed to days that didn’t.

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Based on their findings on temperature data, the researchers warn that by 2090, in the 28 cities where they carried out their research, nighttime temperatures will nearly double — from the current average of 20.38 degrees Celsius, to 39.77 degrees Celsius. Based on the relations they establish between HNEs and mortality rates, the researchers also predict that mortality rates would rise by 60% in these cities. One of the major reasons for that would be the loss of nighttime sleep in hotter climates, which builds up over time and can lead to major health risks.

The study noted that “Ambient heat during the night might interrupt the normal physiology of sleep. The subsequent health effects…are numerous, such as immune system damage, increased susceptibility to cardiovascular disease, chronic illnesses, systemic inflammation, and psychological and cognitive damage.” The idea is that a decrease in core body temperature is crucial for sleep; this means surrounding temperatures shouldn’t exceed a certain level. High ambient temperatures at night may affect the core body temperature, eventually resulting in poor sleep by affecting circadian thermoregulation. The scientists highlight previous studies that have also looked at increases in nighttime temperatures and their relation to sleep quality.

Dr. Haidong Kan, a co-author of the study, told NPR that “to combat the health risk raised by the temperature increases from climate change, we should design efficient ways to help people adapt. Locally, heat during the night should be taken into account when designing the future heatwave warning system, especially for vulnerable populations and low-income communities who may not be able to afford the additional expense of air conditioning.” The scientists acknowledge the limitations of geography in conducting a study across only 28 cities in East Asia, and emphasize that more data on mortality and HNE needs to be audited across the globe.

The study comes at a time when heat waves have increased in frequency and intensity across the globe. Earlier this year, a heat wave in India and Pakistan resulted in the region recording its hottest March since 1901. India reported 280 heatwave days across 16 states between March and May this year, a five-fold increase from the number of such days witnessed in 2021. A spur of heat waves has gripped Europe since June this year. The climate crisis has already affected global agriculture and food supply, and extreme weather events have caused death and destruction to millions. The new study now highlights how a gradual increase in temperatures may also increase regular mortality rates. Even without extreme weather events, with just a slow but steady rise in nighttime temperatures, the loss of life due to climate change can be immense.