rewrite this title For good sleep and good health, regulate your exposure to light

Your daily light exposure impacts your health. A new study finds that too much light at night and not enough natural light during the day can be harmful. This story first aired on Morning Edition on Nov. 4, 2024.

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Diet, exercise and sleep all affect your health. But there’s one more factor, as NPR’s Will Stone reports.

WILL STONE, BYLINE: Light is the most powerful cue for our circadian rhythms. These patterns in our biology fluctuate throughout the day, influencing our hormones, blood pressure, metabolism, when we fall asleep and countless other functions. And for millennia, humans evolved with the natural dark-light cycle.

DANIEL WINDRED: It’s only very recently that we’ve actually been able to manipulate our lighting environments.

STONE: Daniel Windred is a postdoctoral researcher at Flinders University in Australia.

WINDRED: If we have bright nights and dark days, we’re actually altering the way our cells and tissues operate through our body.

STONE: A massive new study from Windred and his team suggests light-driven disruption can take years off our lives. They collected data from close to 90,000 people in the U.K., who each spent a week with a light-sensing device on their wrist. Their analysis shows exposure to light predicted the risk of dying over the next eight years.

WINDRED: We found that people exposed to the brightest nights had a 21- to 34% higher risk of premature mortality.

STONE: On the other hand, bright days were associated with lower mortality – as much as a 34% decreased risk for the top light-getters. Windred says this probably represents people who spend more time outside during daylight.

WINDRED: There’s, like, a massive jump in the intensity between an indoor and an outdoor light environment.

STONE: While the study can’t prove causality, the link between mortality and light was there even when controlling for factors like physical activity and income. Dr. Charles Czeisler is a longtime circadian researcher at the Brigham and Women’s Hospital and Harvard Medical School.

CHARLES CZEISLER: I think that this is a very exciting milestone.

STONE: Reflecting decades of evidence, including carefully controlled lab experiments, showing that inappropriately timed light can be hazardous. It’s linked to mental illness, cancer and especially metabolic and cardiovascular-driven diseases.

CZEISLER: We’re not talking about a marginal change in your risk of death or your risk of heart disease or diabetes. We’re talking about huge increases in risk associated with an easily modifiable factor.

STONE: The study suggests getting bright light in the dead of night, specifically between 2 and 3 a.m., was the most harmful. But Czeisler’s lab and others have shown blue-enriched light – coming from a tablet or a smartphone – even before bedtime can disrupt your circadian rhythms. The good news is that daylight can help synchronize our rhythms and even be protective.

CZEISLER: Exposure to brighter light during the daytime makes us less sensitive to light at night.

STONE: The dangers of the night shift to health are well-documented. This U.K. study excluded shift workers, though, which makes it relevant to many more people. Laura Fonken, a neuroscientist at the University of Texas at Austin, says this study is a good reminder for those of us who spend our days in an office.

LAURA FONKEN: Where you’re in a cubicle where there’s no windows around, you’re also not getting normal daytime light levels. So then it’s really that your body can’t sense that contrast between night and day very well.

STONE: This suppresses the natural ups and downs of our circadian rhythms, which is also linked to higher mortality. The solution here is intuitive – as much as possible, shield yourself from light during the middle of the evening. And during the day, Dr. Czeisler advises people to get outside for at least 30 to 45 minutes.

CZEISLER: They will do wonders for their health.

STONE: It doesn’t have to be in broad daylight, either. A cloudy day, first thing in the morning or late afternoon – all of it can help keep our circadian rhythms on track.

Will Stone, NPR News.

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