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Exposure to Ambient Light at Night May Increase an Individual’s Risk for COVID-19

August 05, 2021
by Michael Greenwood

Locations with significant amounts of artificial nighttime light, such as would be found in developed areas with streetlamps, neon advertising and heavy automobile traffic, have a higher incidence of COVID-19 infection, new research from the Yale School of Public Health finds.

Artificial light at night is known to disrupt a person’s internal circadian rhythms, which in turn negatively affect the human immune system and consequently increase an individual’s susceptibility to infections, including COVID-19.

The study is believed to be the first linking ambient light levels at nighttime with rates of COVID-19 infection.

The researcher team, led by Associate Professor Yong Zhu, Ph.D., of the Yale School of Public Health, analyzed data on positive COVID-19 test rates and nighttime light intensity gathered from widely different settings in four states: New York, Connecticut, California and Texas.

After accounting for potential confounding factors such as population density, poverty and race, the researchers found a strong connection between COVID-19 incidence and the intensity of nighttime light in two of the four states: New York and Connecticut. For example, the city of Bridgeport, with the highest COVID-19 incidence in Connecticut, is among the municipalities with the highest light-at-night (LAN) intensity as well.

“These findings suggest that repeated and prolonged exposure to artificial nighttime light could play a role in COVID-19 incidence,” said Zhu, the study’s lead author. “We are just beginning to understand the role of circadian rhythms on health, and this is further evidence that a connection exists.”

The findings are published in the journal BMC Public Health.

Specifically, the researchers found that the co-distribution between the amount of LAN and COVID-19 incidence was more evident in Connecticut and New York than in Texas and California. This difference might be due to different lockdown and reopening policies in each state. For example, California had a shorter lockdown period, and there were no formal “stay at home” orders in Texas, compared to New York, which has similar total COVID-19 cases and population, but has a much stricter policy.

We are just beginning to understand the role of circadian rhythms on health.

Yong Zhu

In states with strict policies, people spent much more time at home and were far less mobile during the pandemic, including limited travel, recreation and trips to the grocery store. The less-strict policies may have introduced more social factors into the analysis that could not be accounted for in the study. These more complex situations and different social and demographic factors may have masked the correlation between LAN and COVID-19 infection in these states, the authors said.

Circadian rhythms are a biological “clock” that regulates many basic processes, most notably the human sleep cycle. People are wired to sleep after sunset and awake after sunrise due to evolutionary adaptation to 24-hour day-night circadian cycle of the Earth’s rotation to the sun. Disruptions to this cycle have been found to be a significant factor in regulating the immune system, and disruptions can adversely affect immunity against a variety of infections.

Previous studies have shown that circadian disruptions that occur among night-shift workers and others have resulted in increased susceptibility to gut and respiratory infections. Zhu has also found the circadian disruption may be a factor in breast and prostate cancer and lymphomas.

Other authors on the study include M.P.H. student Yidan Meng and summer intern Vincent Zhu in Zhu’s lab. The study was supported by funding from Yale University.


Michael Greenwood at

Submitted by Ivette Aquilino on August 05, 2021