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Having Trouble Sleeping? You Aren’t Alone – And Here’s Why

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Many media outlets portray climate change as representing scorching afternoons, intense droughts, and broken heat records. However, scientists are growing increasingly concerned regarding nocturnal temperatures. Nighttime thermal profiles are currently rising faster than diurnal temperatures in many parts of the world. Nocturnal cooling is incredibly important to humans, animals, plants, and particularly infrastructure, as it allows them recovery from the intense solar heat present in the day. As nightly temperatures rise, this crucial recovery period shrinks. Without ample time to recover from daily heat, wildlife and crops are stressed, human health plummets, energy demand grows, and heat waves are significantly more dangerous. Contrary to common belief, many key dangerous effects of climate change occur at night, an issue many climate scientists are increasingly raising awareness on.

Historically, temperatures substantially drop at night, creating a natural cooling period countless life depends on. Humans cool down while sleeping, plants recover from daytime stress, and buildings release heat accumulated during daylight hours^[¹^,²^]. However, climate change is disrupting this important routine dramatically. Across the globe, nighttime land surface temperatures are rising faster than daytime temperatures^[³^,⁴^]. Consequently, nights are no longer cooling adequately. This reduction is referred to as diurnal temperature range, essentially the difference between daytime highs and nighttime lows^[³^,⁵^]. One primary reason this phenomenon occurs is largely due to the entrapment of greenhouse gases in the atmosphere. Greenhouse gases trap heat independent of whether it is day or night^[⁶^,⁷^]. During the day, solar heat warms the ground. However, at night, greenhouse gases insulate Earth, preventing heat from efficiently escaping into space^[⁶^,⁸^]. Augmented cloud cover amplifies this effect^[⁹^]. Warming temperatures cause increased atmospheric moisture. This moisture creates more nighttime cloud cover, trapping more heat near Earth’s surface.

Video Credit: Nasa/JPL Caltech

For humans, warming nocturnal temperatures particularly increases the risk of heat-related mortality^[¹⁰^,¹¹^]. In order to recover from daytime heat stress after extensive heat waves, bodies rely on cooler nighttime temperatures^[¹¹^,¹⁰^]. However, not only have heat waves occurred more often due to climate change, but the important recovery time at night is rapidly diminishing as well for the same reason. When temperatures remain elevated overnight, human bodies experience prolonged physiological strain, posing a significant cardiovascular risk^[¹¹^,¹²^]. Medical researchers found that high nighttime temperatures were heavily associated with increased heat-related illnesses, higher hospitalization rates, greater cardiovascular stress, poorer sleep quality, and increased mortality during heat waves^[¹⁰^,¹¹^,¹²^,⁴^,¹⁴^,¹^]. Some studies even suggested that unusually warm nights were likely better predictors of heat-related deaths than daytime temps alone^[¹⁰^,¹⁷^]. A commonly disregarded aspect directly related to this is sleep. The human body naturally lowers its core temperature when dormant^[²¹^]. Hot nights interfere with this process, reducing sleep quality and affecting physical and mental health^[¹⁴^,²²^]. Interruption of the body’s most important resting period can lead to severe health risks. Researchers estimate that climate change has already caused billions of hours of lost sleep globally, driven almost entirely from rising nighttime temperatures^[¹⁴^]. However, our health is not the only negative consequence associated with rising nocturnal temperatures.

Agriculture is increasingly being affected by these inconsistent temperatures. Plants create essential sugars during the day through photosynthesis. At night, these plants burn some of these crucial stored sugars as energy through respiration. This process allows the plant to grow, pull essential nutrients and water from the soil into the plant’s tissues, and repair internal cellular damage^[²^]. Higher nighttime temperatures increase respiration rates, meaning that plants are forced to use more stored energy. As maintenance respiration increases with higher nighttime temperatures, the plant is left with less sugars and ultimately energy available for growth and grain production^[²^,¹³^]. Studies have linked warmer nighttime temperatures to reduced yields in essential crops such as rice, wheat, corn, and soybeans^[¹⁵^,¹⁶^]. Rice is one of the most studied examples. Researchers found that increased nighttime temperatures can significantly reduce rice yields originally expected to be plentiful^[¹⁶^,¹⁹^]. This is mainly attributed to the higher levels of energy expended for plant maintenance during warmer nights, leaving less sugars for growth and grain production.

Additionally, wildlife are increasingly finding it harder to survive as temperatures rise. Many animals have evolved around predictable daily temperature cycles. For instance, desert foxes become active during the night primarily due to the cooler temperatures. Many species of bats hunt solely at night, and owls, moths, rodents, and amphibians all depend on cooler nocturnal conditions. Even coral reef species time essential behaviors around daily thermal cycles, meaning that when nighttime temperatures are disrupted, these essential behaviors are mistimed, resulting in devastating consequences for life everywhere. Warmer nights can disrupt feeding patterns, migration timing, breeding behavior, predator-prey relationships, and, crucially, habitat use. Nocturnal species are particularly vulnerable to this rapid warming as their active hours become increasingly hot. However, one of the biggest effects felt from nighttime heating may be coming from our infrastructure itself.

Cities are experiencing the strongest effects of increased nighttime temperatures. Because urban areas are commonly covered in concrete, asphalt, and buildings, they absorb and retain more heat than surrounding rural areas do. This is known as the urban heat island effect^[¹⁸^{, ]}. Concrete, asphalt, brick, and buildings in general absorb high levels of solar energy during the day. Unlike vegetation, these materials store the heat, and slowly release it after sunset. Cities release stored heat slowly after sunset^[¹⁸^]. Because urban areas are warmer than rural areas, urban heat waves are particularly dangerous. Additionally, warmer nights increase electricity demand in unorthodox ways. As overnight temperatures rise, more households and businesses rely on air conditioning throughout the night^[²⁰^]. This increases energy consumption, strains electrical grids, and, importantly, leads to higher emissions, particularly when the electricity is generated from harmful fossil fuels^[²⁰^].

Scientists expect this warming trend to continue throughout the 21st century^[³^,²⁰^]. Climate models project that nighttime warming will remain a major consequence of global climate change, particularly in urban regions, tropical climates, and areas already experiencing frequent heat waves. This warming causes severe concerns regarding wildlife, vegetation, crops and food procurement, and most importantly human well-being. As climate change continues to degrade the cooling time that ecosystems, agriculture, infrastructure and humans depend on as recovery, serious concerns and awareness are being raised^[¹¹^,¹⁰^]. Climate change no longer affects the day alone. As global warming continues to raise unintended consequences, it is more important than ever to raise awareness and understand the true drivers behind these dangers.