New York might be known as the city that never sleeps, but it turns out there's a lot less shuteye happening in urban areas across the globe. And it's not because of nightlife, but the artificial light emitted from buildings and infrastructure that is driving up levels of insomnia for even those in smaller metropolises.
In the latest study and the biggest conducted in the region, Southern University of Science and Technology (SUSTech) researchers looked at social media data and satellite-sourced nighttime light imaging of 336 small, medium and large cities across mainland China. Between May 2022 and April 2023, some 1,147, 583 insomnia-related posts were made on one of the country's most popular social media platforms, Weibo.
For analysis, the team narrowed the data down to people aged 15 to 39 years in each city – largely because this demographic accounts for 96% of Weibo's user base. Using incidence of insomnia, or the total number of insomnia-related posts per 10,000 users, the researchers were able to get a clear picture of how artificial light at night (ALAN) is associated with sleeplessness.
"The incidence of insomnia among residents at the city level was measured by the number of insomnia-related posts on social media," the researchers noted. "Multiple linear regression models were used to estimate the association between ALAN exposure and population insomnia, adjusting for population characteristics and meteorological factors at the city level."
While earlier studies have also found a link between sleeping disorders and urban populations with high exposure to ALAN, they've generally found that the bigger the city, the higher incidence of insomnia experienced by the inhabitants. Here, however, the team discovered more nuances.
"Different from the patterns observed in developed countries, the association between ALAN exposure and insomnia in our study was higher in medium and small cities compared with large cities," they wrote in the paper. "This divergence may be attributed to disparities in ALAN distribution, regional development stages, and urban lighting planning."
This linkage could be explained by China's rapid urbanization, which continues to accelerate. Over the past 75 years, the country has transformed from rural to urban so quickly it's now the largest and fastest transformation in world history. While impossible to say for sure in this study, the researchers suggest that such speedy change without a focus on light pollution could see exposure to ALAN more detrimental to these newer, smaller cities.
While artificial indoors light – such as from screens – has repeatedly been linked to a range of health issues in humans, light pollution from our urban infrastructure is often seen as the silent, less harmful aspect of industrialization. Because of this, we're only just starting to learn how it affects native species of animals and plants. While artificial light can alter plant growth and form, its most insidious impact is how it interrupts natural behaviors of animal species, from birds to mammals. ALAN isn't just moths flying into street lamps, it can also cut off nocturnal foraging, communication, predator-prey relationships and, yes, sleep. It's widely considered one of the fastest growing forms of pollution on Earth.
And humans are not immune from ALAN, with many studies linking artificial night light and its complex impact on our biology.
"Several biological mechanisms have been proposed to explain the association between ALAN exposure and insomnia, including dysregulation of pineal gland function and melatonin secretion, altered neuronal circuits, and activation of oxidative stress genes," the researchers note in the paper. "Melatonin, a hormone produced by the pineal gland, plays a crucial role in regulating sleep-wake cycles and other biological rhythms.
"Some studies have also suggested that long-term exposure to ALAN can activate oxidative stress genes, impacting human cell function and circadian rhythm," the team added.
There are some limitations to this study, however. We don't know the extent of indoor artificial light at night, which has been shown to impact sleep quality, and noise and other factors were not considered. And because social media geotagging could place posters in a general urban location, the study assesses the city's population as a whole, which is quite generalized when you consider the massive range of light exposure across the urban environment.
However, it does bring to light – no pun intended – how, in developing nations, smaller and less illuminated environments don't necessarily mean lower insomnia incidence. And it suggests urban planners can do more for the wellbeing of residents by factoring in ALAN. It also makes a case for the importance of effective blockout blinds …
"Our findings suggest that ALAN may be an important risk factor associated with insomnia, which may guide further research into the potential role of ALAN exposure in sleep issues as a determinant of health disparities," the researchers noted. "Furthermore, this study emphasizes the importance of rational planning and layout of artificial nighttime lighting installations, particularly in small-to-medium cities. Currently, developing countries such as China still lack sufficient outdoor lighting standards and regulations. Thus, implementing local light pollution prevention policies is needed to protect population health benefit along with economic development.
The study was published in the journal JAMA Network Open.
Source: Southern University of Science and Technology via Scimex
