Jin Dai, Wen Dai, Yoriko Heianza and Lu Qi, epidemiologists at Tulane University in New Orleans, put the two measures together. The paper appeared on 9 September 2026 in the European Heart Journal and is open access. There are 11,071 participants, with a mean age of 61.1. Women make up 56.6 per cent and white participants 97.6 per cent.
The threshold is three lux. One lux is roughly the light of a candle one metre away. The authors chose the value because it is enough to suppress melatonin, the hormone that rises as evening darkens. People who never crossed it during sleep form the reference group, 3,438 of them. The rest are split into three equal groups of low, intermediate and high exposure. The high group has 2,416 people. The sections of the paper that could be opened do not give the lux values that separate the three groups.

In the high group, after adjusting for age, habits, air pollution, season and daylight exposure, left ventricular mass is 2.4 per cent greater, with a 95 per cent confidence interval of 1.6 to 3.1. The wall is 1.5 per cent thicker. Myocardial contraction fraction, which measures how much the muscle shortens with each beat relative to its volume, is 1.9 per cent lower. Strain, the deformation of the muscle as it contracts, falls by almost 3 per cent in all three directions in which it is measured. The right ventricle and the left atrium are also slightly larger, and the atrium empties slightly less.
The authors call the pattern concentric hypertrophy with subclinical functional decline: thicker walls, a heart working slightly worse, no symptoms. These are differences of a few percentage points in group averages, and they cannot pick out a single patient. They matter because they all point the same way and rise in a straight line with the dose of light.
Part of the link runs through sleep. People who sleep with more light sleep less, and sleep duration statistically accounts for between 24 and 49 per cent of the associations. Sleep was not measured in a laboratory, though. The same wrist device estimated it.

The Tulane team then followed 73,286 participants who had worn the sensor for eight to ten years. In the brightest group the risk of heart failure was 29 per cent higher, stroke 33 per cent, heart attack 24 per cent and cardiovascular death 26 per cent. These are hazard ratios, which say how much faster events accumulate in one group than in another. Case counts per group are missing from what could be opened. Without them the 29 per cent cannot be turned into extra cases per thousand people.
The authors list their limitations. The recording covers a single week, which may not stand for years of nights. The scan was done on those who agreed to return, a group unlike the starting cohort. The sample is almost entirely white and British. One more point goes undiscussed, at least in the sections available. The sensor sits on the wrist, and a wrist under the covers does not see the light that reaches the eyes.
The study is observational and does not show that light thickens the heart. A lit bedroom often travels with shift work, city life, a screen in bed and sleep broken for other reasons. The authors adjusted for many of these factors. Shift work and screen use do not appear among the listed adjustments. The European Society of Cardiology press release says too much light at night changes the shape and working of the heart. The paper says association.
The work was funded by the US National Institutes of Health. The authors close on two points, dark nights and enough sleep. Proof of cause can only come from a trial that turns the lights off for one group and leaves them on for the other.

References
Dai J., Dai W., Heianza Y., Qi L., “Nighttime light exposure and cardiac structure and function”, European Heart Journal, article ehag563 (volume and issue not yet assigned), published online 9 September 2026, open access under CC BY 4.0. doi: 10.1093/eurheartj/ehag563
European Society of Cardiology, “Too much light at night changes the shape and working of the heart”, press release, 10 September 2026.

*Board Member, SRSN (Roman Society of Natural Science)
Past Editor-in-Chief Italian Journal of Dermosurgery