Falling asleep with the TV glowing, a night-light beside the bed or light coming in from the hallway may seem harmless. New research suggests that even relatively low levels of light during sleep could be linked to changes in the structure and function of the heart.
The study, published September 9 in the European Heart Journal, followed thousands of adults in the UK and found that people exposed to more light at night showed early changes in several chambers of the heart. A separate analysis involving more than 73,000 people also found higher rates of heart failure, heart attack, stroke and atrial fibrillation among those with greater nighttime light exposure.
The results do not prove that leaving a lamp on causes heart disease. They do give researchers another reason to look closely at something most of us rarely think about when we go to bed. How dark is the room once we actually fall asleep?
Researchers Measured Light While People Were Sleeping
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The new European Heart Journal study used data from the UK Biobank, a large research project that has followed the health of hundreds of thousands of people.
More than 11,000 participants wore wrist devices with built-in light sensors for seven days. The sensors recorded how much light each person was exposed to, including during the least active five-hour period of the day, which researchers used to identify the likely sleeping period.
About three years later, those participants underwent detailed heart MRI scans. Researchers then compared heart structure and function with the amount of light recorded during sleep.
The differences were not huge, but they were measurable. People in the high nighttime light group had greater muscle mass and thicker walls in the left ventricle, the main chamber responsible for pumping blood around the body. Researchers also found signs that the heart muscle was not contracting quite as effectively.
Changes were seen in the right ventricle and left atrium as well, which is one reason the study received so much attention. The findings were not limited to a single measurement in one part of the heart.
Three Lux Is Not a Bright Bedroom
One of the most surprising details is how little light researchers were looking at.
The study focused on nighttime exposure above 3 lux. For comparison, CNN reported that experts described 3 lux as roughly comparable to moonlight or light slipping under a closed bedroom door.
So we are not talking only about sleeping under a bright ceiling light.
A television left on across the room, a strong night-light, streetlights coming through thin curtains or light from another room may all contribute to the amount of light reaching someone during the night.
Individual exposure can vary a lot depending on distance, brightness, room layout and where the light is coming from. A wrist sensor also does not measure exactly what reaches the eyes, so 3 lux should not be treated as a strict household danger line.
The Heart Changes Were Small but Consistent
Compared with participants who had no nighttime exposure above 3 lux, those in the high exposure group had several differences on heart MRI.
- Left ventricular mass was 2.4% greater.
- Average wall thickness was 1.5% greater.
- Myocardial contraction fraction was 1.9% lower.
- Researchers also found changes in measures of how the heart muscle deforms during contraction.
- Changes were recorded in the right ventricle and left atrium as well.
A difference of 1% or 2% may not sound dramatic. Researchers were interested because the pattern points toward cardiac remodeling, a term used when the size, shape or function of the heart begins to change over time.
Cardiac remodeling can occur when the heart is exposed to long-term strain or injury. The people in the study were not being diagnosed with heart failure simply because an MRI measurement changed. Researchers were looking for early structural differences that could help explain why nighttime light has been linked to cardiovascular disease in previous studies.
The Longer Follow-Up Found Higher Cardiovascular Risk Too
The research team also analyzed cardiovascular outcomes in 73,286 UK Biobank participants over roughly eight to ten years.
After accounting for a long list of health, lifestyle, social and environmental factors, greater nighttime light exposure remained associated with higher cardiovascular risk.
| Health outcome | Higher risk in the high nighttime light group |
|---|---|
| Heart failure | 29% |
| Atrial fibrillation | 15% |
| Heart attack | 24% |
| Stroke | 33% |
| Death from cardiovascular disease | 26% |
Those percentages describe relative differences between the study groups. They do not mean that someone who sleeps with a night-light has a 29% chance of developing heart failure.
That distinction matters. Relative risk numbers can sound alarming when they are pulled out of a study without explaining what they represent.
Why Would Light at Night Affect the Heart?
Light helps set the internal clock that tells the body when to be awake and when to sleep. Exposure at the wrong time can interfere with that system.
Nighttime light has been linked in earlier research to suppression of melatonin, changes in sleep, blood pressure and disruption of circadian rhythms. All of those systems are connected in some way to cardiovascular health.
The new study found that shorter sleep appeared to explain between 24% and 49% of many of the heart changes associated with nighttime light. In other words, poorer or shorter sleep may be part of the explanation, but it does not appear to explain everything.
We previously looked at why good sleep involves more than simply spending eight hours in bed. Timing, continuity, regularity and sleep quality also matter, which helps explain why researchers are increasingly interested in the environment where sleep takes place.
One Study Does Not Prove That a Night-Light Damages Your Heart
The study is important, but there is a limit to what it can tell us.
Researchers observed what happened to people with different light exposure. They did not randomly assign thousands of people to sleep in bright and dark bedrooms for years. Because of that, the study cannot prove that nighttime light directly caused the heart changes or cardiovascular events.
Light exposure was also measured for seven days and used as an estimate of each participant pattern over time. The devices measured brightness but did not record the exact color or wavelength of the light, which can affect the response of the internal clock.
UK Biobank participants also do not perfectly represent every adult in the United States or elsewhere. The numbers are useful evidence, not a guarantee of what will happen to any one person.
You Probably Do Not Need to Redesign Your Bedroom
The good part is that reducing light at night is one of the easier health habits to try.
An editorial published alongside the study suggested practical changes such as blackout curtains, warmer bedside lighting and covering small standby lights on electronics.
You can also look around the bedroom after turning off the main light. A glowing television, bright alarm clock, charger light or streetlamp shining directly through the window is easy to miss when you are used to seeing it every night.
People who need a night-light for safety do not need to turn the room completely black and risk falling on the way to the bathroom. A dimmer light placed low to the floor can reduce exposure without creating another problem.
Heart Health Still Comes Down to More Than Bedroom Lighting
No one should walk away from this study thinking that darkness can cancel out smoking, high blood pressure, inactivity or poor diet.
Nighttime light is being studied as another possible contributor to cardiovascular risk, not as the main cause of heart disease.
We recently covered research suggesting that small changes in sleep, physical activity and diet may add up to a lower risk of heart attack and stroke. Bedroom light may eventually become another piece of that discussion, especially because changing it costs little or nothing.
So Should You Turn the Lights Off Tonight?
Based on what researchers know so far, sleeping in a darker room is a reasonable choice.
The new study does not show that one night in front of a glowing TV will change the shape of your heart. The concern is repeated light exposure night after night, potentially disturbing sleep and the internal clock over many years.
If you already sleep in a dark room, there is probably nothing new you need to do. If the TV stays on until morning or streetlight pours through the window all night, reducing that light is a simple change worth considering.
Researchers still need controlled studies to find out how much of the cardiovascular risk is actually caused by nighttime light and how much can be reversed by sleeping in darker conditions. For now, turning off a light is one of the easier experiments you can make in your own bedroom, and your electric bill will not complain either.
