Light exposure linked to mortality risk

Key takeaways:

• Those with the highest day light exposure had a 17% to 34% reduced mortality vs. those with the lowest.
• Earlier and later circadian phase and lower circadian amplitude also increased the risk for mortality.

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 light, 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 70^th to 90^th and 90^th to 100^th 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 50^th percentiles.

Meanwhile, those in the 70^th to 90^th percentile of night light exposure had a 22% to 26% increased risk for cardiometabolic morbidity vs. those with the lowest night light exposure, while those in the 90^th to 100^th 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 50^th to 70^th, 70^th to 90^th and 90^th to 100^th 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 50^th 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.”