The Hidden Danger of Nighttime Light for Your Heart

Do you also stay awake in front of screens or with a light on in the bedroom? The study examined nearly 89,000 participants aged 40 and above, measured using wrist worn light sensors for a full week over 13 million hours of data and found a clear dose dependent link between brighter nights and higher risk of cardiovascular diseases, including heart attack, heart failure, and atrial fibrillation.

The data were cross referenced with national health records over almost ten years of follow up, and the conclusion was clear: the brighter the room at night, the higher the heart risk.
The findings were recently published in the medical journal JAMA Network Open.
“The major advantage of this study is its scale both in the number of participants, the duration of follow up, and the precise way the data were collected,” says the lead researcher.

“The findings indicate a clear link: the more people are exposed to light at night, the higher their risk of various heart events.”
The study included 88,905 men and women aged 40 and above, with an average age of 62.
Participants wore wrist light sensors for a week, allowing researchers to accurately map lighting conditions over 24 hours.

Time was divided into two main windows: “day” from 07:30 to 20:30, and “night” from 00:30 to 06:00.
The brighter the nights, the higher the risk of major heart diseases. Participants were divided into four exposure levels from the darkest nights (up to 50%) to the brightest nights (91%-100%) and the differences were sharp.

Compared to dark nights, those spending nights under stronger light had a 23% higher risk of coronary heart disease, 42% higher risk of myocardial infarction and 45% higher risk of heart failure.
The risk of atrial fibrillation and stroke also increased by approximately 28% in each case.

Researchers also found a continuous and clear correlation: even small increases in nighttime light intensity even within normal ranges were associated with a measurable increase in risk for each of the five diseases studied.
Women at Higher Risk Humans, like all other living organisms and plants, are synchronized with light and dark cycles.

All biological and physiological systems are built around this pattern. Artificial light and modern life disrupt this cycle, causing biological disruptions.
Shift work or prolonged exposure to artificial light can be harmful.
Exposure to screens, particularly smartphone screens, acts like a direct spotlight on the eyes and disrupts internal systems.
In this case, the effect was negative on the cardiovascular system.

The effect was not uniform across all groups.
In women, the link between nighttime light and coronary heart disease and heart failure was especially pronounced.
Younger participants showed greater sensitivity to heart failure and atrial fibrillation.
Even accounting for genetic risk, nighttime light remained an independent and significant factor.

Screen Use Disrupts Sleep Nighttime light suppresses melatonin production, the hormone secreted by the pineal gland in the brain.
Melatonin regulates the body’s response to light dark cycles, maintains biological clock synchronization, and supports sleep quality and normal body function.

Daytime Light Exposure to more light during the day was linked to lower risk of coronary heart disease, heart failure, and stroke.
However, after adjusting for lifestyle factors, mainly physical activity, the protective effect weakened.
When physical activity was removed from the analysis, beneficial effects remained mostly for heart failure and stroke.

How It Was Measured Participants wore wrist light sensors for a week, which collected continuous data and divided the day into 48 half hour segments. Health outcomes were measured via hospitalization records, GP visits, mortality records, and self reports, with follow-up through November 2022.

Differences between groups were dramatic: the dark night group averaged 0.6 lux almost total darkness while the brightest group recorded over 100 lux, similar to a room with lights on.
Not Just Short or Poor Sleep Short or inefficient sleep did not fully explain the phenomenon.

Researchers controlled for sleep duration and quality, and most links remained significant.
Short sleep weakened some associations but did not eliminate them.
Only in the case of stroke was there a slight reduction in statistical significance.

The Biological Clock and Heart Health As in all studies, there were limitations. Participants do not fully represent the entire population they were mostly white, with above-average education and income. Light measurement was done only for one week, so it is unknown if exposure levels changed over the years.