Most of us think about sleep in terms of hours.
Seven hours. Eight hours. Maybe six on a bad night.
But growing research suggests there may be another part of sleep hygiene worth paying attention to:
how dark your bedroom actually is.
A recent study published in the European Heart Journal found that greater exposure to light at night was associated with measurable changes in heart structure and function—and with higher rates of several cardiovascular problems over time.
That does not mean a bedside lamp will suddenly cause a heart attack.
The research is observational, so it cannot prove that nighttime light directly causes cardiovascular disease.
But it adds to a growing body of evidence suggesting that the human body may function best when daytime is bright and nighttime is genuinely dark.
What Did the New Study Find?
Researchers analyzed data from the UK Biobank.
In one part of the study, more than 11,000 adults wore wrist devices that measured their exposure to light over seven days. Years later, they underwent detailed cardiovascular magnetic resonance imaging.
The researchers found that higher nighttime light exposure was associated with several subtle changes in heart structure and function, including greater left ventricular mass, thicker heart walls and reduced measures of cardiac contraction.
A larger analysis involving more than 73,000 participants then examined long-term cardiovascular outcomes.
Compared with people with essentially no nighttime exposure above 3 lux, those with high exposure had higher observed risks of:
- Heart failure
- Atrial fibrillation
- Myocardial infarction
- Stroke
- Cardiovascular death
In this analysis, high nighttime light exposure was associated with about a 24% higher risk of myocardial infarction, a 33% higher risk of stroke, a 29% higher risk of heart failure, and a 26% higher risk of cardiovascular mortality.
Those percentages describe statistical associations between groups.
They do not mean that keeping a lamp on raises every individual’s personal heart attack risk by exactly 24%.
That distinction is important.
How Little Light Are We Talking About?
Potentially less than you might imagine.
The study examined exposure above roughly 3 lux at night.
For perspective, that is far dimmer than typical indoor daytime lighting.
Nighttime light can come from many sources:
- Streetlights shining through curtains
- Hallway lights
- Televisions
- Bedside lamps
- Bathroom lights
- Phone and tablet screens
- Electronic displays
- Light from another room
- Outdoor security lighting
The concern is not simply whether the light seems bright enough to wake you.
The body’s circadian system can respond to light even while you are asleep.
Why Would Light at Night Affect the Heart?
The answer may lie in your circadian rhythm.
Humans evolved around a strong natural contrast:
Bright during the day. Dark during the night.
Light is one of the body’s most powerful environmental signals.
It helps regulate the internal 24-hour clock that influences:
- Sleep and wakefulness
- Melatonin production
- Blood pressure
- Heart rate
- Metabolism
- Hormone secretion
- Body temperature
- Autonomic nervous system activity
When light enters the eyes at night, it can send the brain a biological message that daytime has not fully ended.
That may interfere with normal nighttime physiology.
Melatonin May Be Part of the Story
Melatonin is often described simply as the “sleep hormone,” but its role is broader.
The brain normally begins releasing more melatonin as evening darkness arrives.
Light exposure at night can suppress or delay that signal.
This is one reason sleep experts frequently recommend dimming lights in the evening and avoiding excessive bright-screen exposure before bed.
Researchers studying nighttime light and cardiovascular health suspect that repeated disruption of the circadian system may influence cardiovascular regulation over time.
But melatonin is unlikely to be the entire explanation.
Your Blood Pressure Is Supposed to Dip at Night
A healthy cardiovascular system normally enters a quieter state during sleep.
For many people, blood pressure drops by roughly 10% to 20% compared with daytime values.
This is known as nocturnal dipping.
Circadian disruption, fragmented sleep and nighttime stimulation may interfere with that normal pattern.
Repeatedly losing part of the body’s nightly recovery period could theoretically increase cardiovascular strain.
Researchers are still working out how much nighttime light contributes independently from other sleep-related factors.
Shorter Sleep May Explain Part of the Association
One particularly interesting finding from the recent study was that shorter sleep duration appeared to explain part—but not all—of the relationship between nighttime light exposure and heart changes.
Researchers estimated that sleep duration mediated approximately 24% to 49% of several observed associations.
In other words, people exposed to more light at night may partly experience worse cardiovascular outcomes because their sleep becomes shorter or less restorative.
But sleep alone did not appear to explain everything.
That raises the possibility that nighttime light may influence circadian biology through additional pathways.
This Is Not the First Study to Raise Concerns
The new research fits into a broader scientific pattern.
A large 2025 prospective cohort study involving 88,905 adults found that people exposed to the brightest nights had higher rates of coronary artery disease, myocardial infarction, heart failure, atrial fibrillation and stroke compared with participants experiencing darker nights.
Even after researchers adjusted for multiple established cardiovascular risk factors—including smoking, diet, physical activity, alcohol intake, sleep duration, socioeconomic status and genetic risk—the associations remained.
Separately, earlier research in older adults in Hong Kong linked greater outdoor nighttime light exposure with higher rates of coronary heart disease hospitalization and death. The researchers specifically cautioned, however, that their findings should not automatically be interpreted as proof of causation.
A 2024 systematic review and meta-analysis involving more than one million participants also found associations between nighttime light exposure and several cardiometabolic conditions.
Daytime Light May Matter Just as Much
There is another interesting side to this story.
The goal may not simply be:
less light.
Instead, it may be:
more contrast between day and night.
A 2026 U.S. study involving 7,607 adults found that people with a greater difference between bright daytime exposure and nighttime light exposure had a lower risk of cardiovascular mortality during follow-up.
That fits with what we know about circadian biology.
Your body benefits from strong environmental cues:
Bright mornings and daytime.
Dim evenings.
Dark nights.
Modern life often gives us the opposite.
We spend the daytime indoors under relatively weak artificial lighting, then remain surrounded by screens and electrical light late into the evening.
Is Your Phone the Main Problem?
Phones get much of the blame, but they’re only one piece of the puzzle.
A brightly illuminated phone held close to the face can certainly provide significant evening light exposure.
But bedroom lighting, televisions, hallway lights and outdoor street lighting may also matter.
And when it comes to circadian effects, both brightness and timing are important.
Reducing screen brightness can help, but sleeping in a genuinely dark room may be just as relevant.
7 Simple Ways to Make Your Bedroom Darker
Fortunately, reducing unnecessary nighttime light is relatively simple.
1. Turn off lights you don’t need
You do not need to sleep in total darkness at all costs, but avoid leaving bright room lights on unnecessarily.
2. Use blackout curtains
If streetlights or neighboring buildings illuminate your room, blackout curtains can make a substantial difference.
3. Cover bright electronics
Chargers, clocks, televisions and routers can produce small but persistent points of light.
Covering or dimming them may help.
4. Keep nighttime lighting warm and dim
If you need to get up during the night, use the lowest practical level of illumination.
Bright overhead lighting can be particularly stimulating.
5. Reduce screen exposure before bed
Phones, tablets and laptops provide both light and psychological stimulation.
Giving yourself a low-light transition period before sleep can support circadian signaling.
6. Get more natural light during the morning
Morning daylight helps reinforce the body’s internal clock.
Even spending some time outdoors after waking can provide a stronger daytime signal than ordinary indoor lighting.
7. Consider an eye mask
If changing the bedroom environment is difficult—for example, while traveling—an eye mask can be a simple solution.
Don’t Turn This Into Another Sleep Obsession
Sleep health advice can sometimes create unnecessary anxiety.
You do not need to panic because you once fell asleep with the television on.
And you should not assume that every small LED light is damaging your heart.
The research looks at patterns of exposure over time.
Cardiovascular health is influenced by many much stronger and better-established factors, including:
- Smoking
- Blood pressure
- Cholesterol
- Diabetes
- Physical inactivity
- Diet
- Obesity
- Alcohol intake
- Genetics
- Sleep disorders
Reducing nighttime light should be viewed as a potentially useful additional habit, not a replacement for established heart-health measures.
What the Study Cannot Tell Us
This is the most important scientific limitation.
The new study is observational.
Researchers can measure light exposure, follow participants and identify statistical associations.
But they cannot prove from this design alone that nighttime light directly caused the heart changes or cardiovascular events.
People exposed to more nighttime light may differ in other ways.
They might:
- Live in denser urban environments
- Experience more traffic noise
- Be exposed to greater air pollution
- Work irregular hours
- Sleep differently
- Have different occupational patterns
Researchers attempted to adjust for many potential confounders, but no observational study can eliminate every possible alternative explanation.
This is why the safest wording is:
Nighttime light exposure has been associated with higher cardiovascular risk.
Not:
Sleeping with a light on causes heart attacks.
The Bigger Lesson: Darkness Is Part of Sleep Hygiene
We spend enormous amounts of money trying to improve sleep.
Supplements.
Special mattresses.
Smart watches.
Cooling systems.
Sleep apps.
Herbal teas.
Yet one of the oldest sleep signals is completely free:
darkness.
Your nervous system has been using light and darkness to regulate its internal clock long before bedrooms, smartphones or electric lamps existed.
Creating a darker sleeping environment is inexpensive, low-risk and consistent with what we currently understand about circadian physiology.
And unlike many wellness trends, you do not need to buy anything complicated to try it.
The Bottom Line
A growing body of research suggests that regular exposure to light during the biological night may be associated with poorer cardiovascular health.
The latest study linked greater nighttime light exposure with structural and functional changes in the heart as well as higher observed rates of heart attack, stroke, heart failure, atrial fibrillation and cardiovascular mortality.
But the research does not prove that nighttime light directly causes these conditions.
For now, the practical message is refreshingly simple:
Get plenty of light during the day. Dim your environment in the evening. And when it’s time to sleep, let the night actually be dark.
It may be one of the easiest ways to give your body’s natural clock a clearer signal.
