Sleeping With Light On May Affect Heart Health, Study Finds
New research suggests that sleeping in low levels of light may affect heart structure and function. Learn about the possible risks and how to create a darker bedroom.
A little light in the bedroom may seem harmless. A glowing alarm clock, a smartphone screen on the nightstand, streetlights coming through the curtains or a hallway light under the door can all appear too insignificant to affect your health.
However, new research suggests that even relatively low levels of light during sleep could be associated with changes in the structure and function of the heart.
The study, published in the European Heart Journal, examined more than 11,000 adults from the UK Biobank who did not have cardiovascular disease when the research began. Researchers found that people exposed to higher levels of light during sleep showed changes in their heart muscle that could potentially be linked to a greater risk of future cardiovascular problems.
The findings do not prove that sleeping with a light on directly causes heart disease. However, they add to growing evidence that nighttime light exposure may influence cardiovascular health and the body's natural sleep-wake cycle.
Why nighttime light could matter for your heart
Our bodies follow a roughly 24-hour biological clock known as the circadian rhythm. Light is one of the most important signals that helps regulate this internal clock.
Bright daylight in the morning helps tell the brain that it is time to be awake. At night, darkness supports the biological processes associated with sleep.
When artificial light remains present during the night, particularly when it is bright enough to reach the eyes, it may interfere with these natural signals.
Researchers have increasingly investigated whether this disruption could affect more than sleep quality. Nighttime light exposure has been associated in previous research with factors such as disrupted sleep, altered blood pressure and changes in metabolism.
The latest study adds another potential concern: changes in the heart itself.
What the new study found
Researchers analyzed nighttime light exposure among more than 11,000 UK Biobank participants. Each participant wore a wrist-based sensor for seven days to record their exposure to light.
Approximately three years later, participants underwent cardiovascular magnetic resonance imaging, commonly known as a cardiac MRI. This allowed researchers to examine the structure and function of their hearts.
The researchers reported that people exposed to more than 3 lux of light during sleep had thicker walls in the left ventricle, one of the heart's main pumping chambers.
The left ventricle pumps oxygen-rich blood throughout the body. Changes that make the heart muscle thicker or less flexible can potentially interfere with how efficiently the heart fills and pumps blood.
The study also identified reduced flexibility of the heart muscle, which researchers described as an early indicator of possible heart dysfunction.
Importantly, the researchers observed a relationship between greater nighttime light exposure and these cardiac changes. That does not establish a direct cause-and-effect relationship.
What does 3 lux actually mean?
The word "lux" describes the amount of illumination reaching a surface.
For comparison, experts cited in the research have suggested that bedrooms should ideally be very dark during sleep, with illumination around 1 lux or less.
A level of approximately 3 lux can be comparable to moonlight or a small amount of light entering a bedroom from elsewhere in the home.
This means the concern is not necessarily limited to sleeping beneath a bright ceiling lamp. Smaller sources can contribute to nighttime exposure too.
Potential sources include:
- A smartphone screen left face-up
- A brightly illuminated digital alarm clock
- Streetlights shining through windows
- Hallway lighting
- Night-lights
- Television or computer screens
- Decorative lighting around the bedroom
The researchers reported that the changes became more pronounced as nighttime light exposure increased.
Sleep duration may be part of the explanation
One interesting aspect of the research is the possible connection between nighttime light, sleep duration and cardiovascular health.
The study authors estimated that between 24% and 49% of the association between nighttime light exposure and the observed heart changes could be explained by shorter sleep duration.
In other words, light may not be affecting the heart through a single pathway.
If artificial light causes someone to sleep less or experience poorer-quality sleep, that reduced sleep could contribute to cardiovascular changes. Light may also influence biological processes involving blood pressure, heart rate, stress and other cardiovascular risk factors.
This is an important distinction because improving sleep habits may be one practical way to reduce some of the potential effects associated with nighttime light.
Does sleeping with a light on cause heart disease?
Not necessarily.
This is one of the most important points to understand about the research.
The study was observational. Researchers identified an association between nighttime light exposure and certain cardiac characteristics, but they did not conduct a controlled experiment showing that light exposure directly caused the changes.
The researchers also relied on a wrist-worn sensor to estimate nighttime light exposure. That measurement may not perfectly represent the amount of light actually reaching a person's eyes.
Additionally, nighttime light was measured for only one week, while the heart scans took place approximately three years later.
Independent experts therefore noted that additional research is needed to determine whether nighttime light directly contributes to cardiovascular disease.
The findings should be viewed as a warning sign for further research rather than proof that a small bedroom light will cause heart failure.
How to make your bedroom darker
The good news is that reducing nighttime light exposure is relatively simple and inexpensive.
Start by identifying the brightest sources of light in your bedroom. Turn off unnecessary lamps, televisions and electronic displays before going to bed.
If outdoor light enters through the windows, blackout curtains or shades can help create a darker sleeping environment.
You can also reposition digital alarm clocks so that their displays face away from the bed. If you use a phone as an alarm, consider placing it farther away and keeping the screen turned downward.
For people who need a night-light, a dim light positioned close to the floor may be preferable to a bright light at eye level. Warm, amber or reddish lighting can also be less disruptive to nighttime biological signals than bright, cooler light.
A comfortable sleep mask can provide another simple solution when controlling the light in the surrounding environment is difficult.
At the same time, darkness at night should not mean avoiding daylight during the day. Exposure to natural light, particularly earlier in the day, is an important signal for maintaining a healthy circadian rhythm.
The bigger picture
The new research adds to a broader conversation about how modern environments may affect cardiovascular health.
Artificial lighting has made it possible to remain active around the clock, but the human body still relies heavily on natural patterns of light and darkness.
For now, the evidence does not mean everyone needs to sleep in complete darkness or worry about every tiny source of illumination.
Instead, the research suggests that reducing unnecessary nighttime light may be a sensible part of healthy sleep habits, especially when combined with other established cardiovascular strategies such as getting adequate sleep, exercising regularly, eating a balanced diet, avoiding tobacco and managing blood pressure and cholesterol.
A darker bedroom is a relatively easy change to make. And while more research is needed to determine exactly how nighttime light affects the heart, creating a sleep environment that supports the body's natural circadian rhythm may be a worthwhile step toward better overall health.
Sources
- Qi L, et al. Research published in the European Heart Journal, 2026, examining nighttime light exposure and cardiac structure and function among UK Biobank participants.
- UK Biobank, a large-scale biomedical database containing health and genetic information from participants in the United Kingdom.
- CNN Health, published September 9, 2026.
Medical Disclaimer
This article is provided for general informational and educational purposes only. It is not medical advice, a diagnosis or a substitute for consultation with a qualified healthcare professional. The study discussed here found an association between nighttime light exposure and changes in cardiac structure and function, but it does not establish that sleeping with light directly causes heart disease. If you have concerns about your heart health, sleep, blood pressure or other symptoms, speak with a qualified healthcare professional.