Light at night went with a thicker, stiffer heart, in 11,071 UK Biobank adults with heart scans

Streetlights, screens and lamps keep bedrooms lit long after dark. In wrist-sensor data followed by heart MRI scans, the brightest nights went with subtle changes in heart structure and function.

An unmade bed beside a large window looking out on lit city buildings

Summary

An observational study of 11,071 UK adults who wore a light sensor for a week, then had heart MRI scans three years later.
People with the most light at night had a 2.4% heavier main pumping chamber and thicker walls.
Measures of how well the heart squeezed and stretched were slightly worse, in a dose-response pattern.
In 73,286 people followed 8 to 10 years, the brightest nights went with 29% more heart failure and 33% more stroke.
A quarter to half of the link ran through shorter sleep, and light at night tracks with many other habits.

Bedrooms are rarely dark any more. A streetlight through thin curtains, a charging phone, a hallway lamp left on: all of it registers on the skin and the eyes long after bedtime. Researchers have linked bright nights to heart disease before, but nobody had looked at what the heart muscle itself looks like in people who sleep in light.

A study in the European Heart Journal paired a week of wrist-sensor light readings with detailed heart scans taken three years later. The brightest nights went with a slightly heavier, thicker, less springy heart.

Why darkness matters for the heart

MedlinePlus describes sleep as active work, not a shutdown: while you are sleeping your brain and body functions are still active, and during each stage, your brain waves, breathing, heart rate, and body temperature change throughout the night. One of the things sleep does, it notes, is give your heart and blood vessels a rest.

Light is the main signal that sets the body clock, so light in the bedroom can shift the timing and depth of that rest. As the authors put it, nighttime light exposure has emerged as a potential risk factor for cardiovascular disease (CVD), but its effect on the structure of the heart was unknown.

How the nighttime light study worked

The participants wore a wrist device with a light sensor for a week between 2013 and 2015. About three years later, 11,071 of them had cardiovascular magnetic resonance (CMR) imaging, a scan that measures the size, thickness and movement of each heart chamber in detail.

The researchers compared people by how much light above 3 lux they were exposed to at night, roughly the level of a dim night light. They also followed a much larger group, 73,286 people, for 8 to 10 years to see who developed heart disease.

What the heart scans showed

The differences were small but consistent. High nighttime light exposure went with 2.4% greater LV mass indexed to height, meaning the left ventricle weighed more for a person’s size, and 1.5% greater mean wall thickness, a pattern the authors call concentric hypertrophy: a thicker, stiffer main pumping chamber.

Function shifted too. Measures of how strongly the heart muscle squeezed and stretched were slightly lower, including 1.9% lower myocardial contraction fraction, meaning the muscle shortened slightly less with each beat, and the right ventricle and left atrium were slightly larger. All associations exhibited linear dose-response relationships: the brighter the nights, the bigger the difference.

Why bedroom light may be worth taking seriously

Alone, a 2% difference in heart muscle mass means nothing to any individual. What makes it interesting is what happened later in the larger group: participants with high nighttime light exposure had 29% higher risk of heart failure, 15% higher atrial fibrillation, 24% higher heart attack, 33% higher stroke and 26% higher cardiovascular death.

Part of that ran through sleep. Between 24% and 49% of the structural associations were explained by shorter sleep duration, leaving a portion that was not. Whether the rest reflects the body clock, stress hormones or something else is not settled, but darkness is cheap and low-risk to arrange.

What an observational light study cannot prove

People who sleep in brighter rooms differ from those who do not. They are more likely to work shifts, live on busy streets, use screens late, or sleep poorly for other reasons, and although the analysis adjusted for many factors, it cannot remove them all.

Light was recorded for a single week and treated as typical of years afterwards. The heart scans came from one time point, so this shows differences between people rather than changes within a person. UK Biobank volunteers are healthier and more affluent than the general population, and the analysis cannot separate light landing on the eyes from light in the room.

What this changes about light in the bedroom

Nothing here calls for a gadget. Blackout curtains, an eye mask, turning off a hallway light and keeping screens out of the bedroom are all free or nearly so, and they line up with standard sleep advice.

The heart-related steps with proven benefit have not changed: blood pressure, smoking, activity, weight and cholesterol. A dark bedroom is a sensible addition to that list, not a replacement for any of it.

People also ask

What did the study find?

Compared with people with no measured nighttime light above 3 lux, those with high exposure had 2.4% greater left ventricular mass indexed to height, 1.5% greater mean wall thickness and 1.9% lower myocardial contraction fraction, meaning the heart muscle shortened slightly less with each beat, along with lower strain measures. Over 8 to 10 years, high exposure went with 29% higher risk of heart failure, 15% higher atrial fibrillation, 24% higher heart attack, 33% higher stroke and 26% higher cardiovascular death.

How was light at night measured?

By a sensor built into a wrist-worn activity monitor, recording light levels for seven days. The researchers counted exposure above 3 lux, which is roughly the brightness of a dim night light.

What is the left ventricle?

The heart's main pumping chamber, which pushes blood out to the body. When it thickens and stiffens, it fills less easily, which over time can lead to heart failure.

Is the effect large?

Individually, no. A 2.4% difference in heart muscle mass is small and would not be noticed by a person. What gives it weight is that it tracked with light exposure step by step and lined up with more heart disease later.

Is it the light itself or the lost sleep?

The analysis suggests both: between 24% and 49% of the link ran through shorter sleep, leaving a share not explained by sleep length.

What can I do about light at night?

Keep the bedroom dark, use blackout curtains or an eye mask if street light comes in, and dim lights in the hour before bed. These are low-risk habits worth trying. This is general information rather than medical advice.