Light exposure linked to mortality risk

Individuals experiencing more frequent exposure to light during nighttime hours may have a higher risk for death, results from a retrospective cohort analysis published in PNAS showed.

Meanwhile, researchers reported a converse link suggesting a decrease in mortality risk among those who had longer exposure to light during daytime hours.

The findings support seeking day light and minimizing night life, the researchers noted.

The disruption of circadian rhythm from light exposure can lead to various negative health outcomes, Daniel P. Windred, from Flinders University in Australia, and colleagues wrote, but whether personal day and night light exposure could predict the risk for mortality has not been thoroughly assessed.

In the study, the researchers evaluated over 13 million hours of light exposure data recorded by wrist sensors worn byt 88,905 U.K. Biobank participants over the course of a week.

The analysis revealed 3,750 deaths in the study cohort over a follow-up period of 8 years, 798 of which experienced cardiometabolic deaths.

Windred and colleagues found that people in the 70th to 90th and 90th to 100th percentiles of night light exposure had a 15% to 17% and 21% to 34% increased risk for all-cause mortality, respectively, vs. those in the 0 to 50th percentiles.

Meanwhile, those in the 70th to 90th percentile of night light exposure had a 22% to 26% increased risk for cardiometabolic morbidity vs. those with the lowest night life exposure, while those in the 90th to 100th percentile had a 33% to 46% increased risk.

The researchers explained that these findings may be due to night light exposure disrupting circadian rhythms, which could lead to cardiometabolic outcomes like stroke, diabetes and obesity and subsequently death.

They also pointed out that disrupted circadian rhythms predicted mortality, as each standard deviation reduction in circadian amplitude corresponded with a 4% to 10% higher risk for all-cause mortality risk and a 7% to 13% higher risk for cardiometabolic mortality.

People in the 50th to 70th, 70th to 90th and 90th to 100th percentiles of day light exposures had a 10% to 16%, 16% to 26% and 17% to 34% reduced risk for all-cause mortality, respectively, compared with those in the 0 to 50th percentiles.

This relationship may be the result of the enhancing effects of day light on circadian rhythm, “which protect against the negative health effects of circadian disruption,” Windred and colleagues wrote.

“Co-occurrence of physical activity with day light exposure is another plausible explanation for relationships of day light with mortality risk,” they added.

The researchers acknowledged some study limitations. The study cohort comprised mostly white individuals — making the general applicability of findings’ to other diverse populations unclear — although it is possible the associations had unmeasured factors.

Ultimately, the data “demonstrate the importance of maintaining a dark environment across the late night and early morning hours, when the central circadian pacemaker is most sensitive to light, and seeking bright light during the day to enhance circadian rhythms,” Windred and colleagues wrote. “Protection of lighting environments may be especially important in those at risk for both circadian disruption and mortality, such as in intensive care or aged-care settings.”

Roger Seheult, MD

I was pleasantly surprised [with the findings] because I thought that the results of the study confirmed everything that we’ve known. We’ve suspected for some time that, based on epidemiological data, light at night was bad because, among other things, it disrupts the circadian rhythm but also shuts down melatonin production from the pineal gland, which we known is beneficial as an antioxidant and other things. There’s also emerging evidence over the last decade that’s pretty strong [that shows] that sunlight during the day also has a lot of beneficial effects. What we didn’t have what was the individual, intervention prospective data that we have now have with this trial. What we had before was epidemiological data based on questionnaires and behavior. I think this is a step forward.

I believe that we need to advise our patients, especially those that are having some of these chronic illnesses that we’re seeing today and having difficulty with sleep, that they need to turn off lights after 9 o’clock and need to get outside and get as much light as possible. The reason why I say outside, even though the study investigators didn’t differentiate that — they just measured light — is that there is a tremendous [greater] magnitude of light outside than there is inside.

[W]ith any good study, there are more questions that are raised, and the same is true for this study. This study measured light that was coming through a wearable watch that they had for one week, and they were able to make these determinations from that one week. But what we don’t know is what type of light was doing the heavy lifting in terms of the mortality reductions during the day. Was it visible light, was it infrared light, was it ultraviolet light? That would go a long way to telling us what kind of behavior we [should be doing] because there’s a big difference between turning off the artificial lights in your house during the day and going outside into the sunlight. There’s a number of health reformers from the 19th century that actually noticed this and commented on the benefits of sunlight during the day and turning off lights at night, so we’ve known about this for some time, but the science is catching up.

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Windred D, et al. Proc Natl Acad Sci U S A. 2024;doi:10.1073/pnas.2405924121.

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