After examining 13 million hours of light exposure data, researchers found that experiencing bright light during the dark hours can significantly increase the risk of heart failure and heart attack. The effect was most notable in younger adults.
While previous research has hinted that sleeping in a room that is evenly moderately lit can harm heart health, researchers from Flinders University in Australia say that the study they've completed is the largest ever to examine how light exposure at night can impact the cardiovascular system.
To carry out their work, they followed 88,905 people aged 40 and older for 9.5 years. All of the participants were taking part in the UK Biobank project, a massive compilation of medical data from half a million volunteers. The study subjects wore sensors on their wrists to track the amount of light they were exposed to daily.
After analyzing the more than 13 million hours of light exposure data, comparing them with medical records, and adjusting for heart-health influencers like exercise, diet, and genetics, they found that those with the brightest light exposure during the nighttime hours of 12:30-6:00 AM had a 56% higher chance of heart failure and a 47% increased likelihood of having a heart attack than those with the least light exposure.
"This is the first large-scale study to show that simply being exposed to light at night is a strong and independent risk factor for heart disease," says lead study author Daniel Windred. "Disrupting your body’s internal circadian clock by repeatedly exposing yourself to bright light at night, when it would typically be dark otherwise, will put you at a higher risk of developing dangerous heart issues."
To classify light exposure, the researchers broke the cohort into percentiles based on brightness levels measured in lux, a standard unit of light intensity. A dark night sky, for example, measures under 0.01 lux, a room with a small lamp on might come in around 5 lux, and a very well-lit room could exceed 100 lux. Those who had a median nighttime light exposure of 0.62 lux were placed in the lower 50th percentile. Those with a median exposure of 105.3 wound up in the 91st-100th percentile and had the worst heart health outcomes.
The study found a stronger correlation between nighttime light exposure and cardiovascular damage in the younger members of the cohort as well as among women.
"Women may be more sensitive to the effects of light disrupting their body clock, which supports earlier research findings," says senior co-author, Sean Cain. "In fact, women exposed to high levels of night light had similar heart failure risks to men, which is unusual because women typically have some natural protection against heart disease."
The reverse is true, too
Interestingly, the researchers also found that those who spent the most time in bright light during the daytime saw protective benefits to their cardiovascular systems. Before adjusting for lifestyle variables they saw that bright daylight exposure lowered the risk of coronary artery disease by 13%, heart failure by 28% and stroke by 27%. After accounting for lifestyle variables like exercise, the effects weakened a bit as, for example, more active people tend to spend more time outdoors.
Still, the study supports previous research done by Flinders scientists in which they found that people who got the most light exposure during their day had a 17-34% reduction in their risk of death while those who had the highest levels of nighttime light exposure saw a 21-34% increase. It also fits with another study carried out by the university showing that late-night light exposure can increase the risk of type 2 diabetes by up to 67%.
The researchers are now calling for further investigations and guidelines into nighttime light usage and levels in homes, hospitals and cities to try to limit its impact on our cardiovascular systems.
"We need to take our body clocks seriously," concludes Professor Cain. "Protecting our natural sleep rhythms could be a powerful way to fight heart disease."
The team's study has been published in JAMA Network Open.
Source: Flinders University via EurekAlert
