A bedroom that feels comfortably dim may seem like a small detail when it is time to sleep. But new research suggests that the amount of light surrounding you at night could be linked to changes happening deep inside the heart.A study of more than 11,000 people has found an association between greater exposure to light at night and changes in the structure and function of the heart. Researchers observed thicker heart muscle, reduced flexibility and other signs that the heart may be adapting to prolonged stress.The findings, published in the European Heart Journal, add to growing research into the possible health effects of light pollution and nighttime light exposure.The study does not prove that sleeping in a bright room directly causes heart disease. However, researchers say the results raise an important question about an often-overlooked part of the sleep environment: how dark the bedroom actually is.Scientists tracked how much light people encountered at nightResearchers led by Professor Lu Qi of Tulane University analysed data from participants in the UK Biobank, one of the world's largest long-term health research databases.The participants wore wrist-mounted light sensors for seven days. The devices provided researchers with an objective measure of how much light people were exposed to during both daytime and nighttime hours.About three years later, the participants underwent cardiac magnetic resonance imaging (MRI). These scans allowed researchers to examine the heart in considerable detail, including the size and structure of its chambers and how the heart muscle was functioning.The team compared people experiencing higher nighttime light levels with those exposed to very little light.A threshold of more than three lux was used to distinguish higher exposure from almost complete darkness.The researchers then looked for differences in the hearts of people in the two groups.What too much nighttime light appeared to do to the heartThe most notable findings involved the heart's ventricles and atria.People exposed to higher levels of light at night were more likely to show thickening of the wall of the left ventricle, the chamber responsible for pumping oxygen-rich blood around the body.When the ventricular wall becomes thicker, the chamber itself can have less room to fill with blood. Researchers also found evidence that the heart muscle was not relaxing and stretching as normally during each heartbeat.Changes were detected in other parts of the heart as well, including the right ventricle and left atrium.These findings are important because changes in the structure of the heart can sometimes develop as the organ responds to prolonged stress or other forms of strain.The researchers refer to this process as cardiac remodelling.It does not mean that everyone exposed to light at night will develop heart disease. Rather, the study found a statistical association between greater nighttime light exposure and measurable differences in cardiac structure and function.That distinction is important because the study was observational.Why light at night could affect cardiovascular healthScientists have increasingly been examining artificial light as a potential environmental factor affecting human health.Our bodies are naturally programmed to respond to the daily cycle of light and darkness. Exposure to bright light after sunset can interfere with the body's normal biological rhythms, including processes involved in sleep and circadian regulation.Nighttime light can come from many sources. Streetlights can shine through windows, while televisions, smartphones, computer screens, bedside lamps and electronic devices can keep bedrooms from becoming completely dark.Researchers have previously linked excessive nighttime light exposure with several cardiovascular risk factors. The new study adds a different piece of evidence by examining the physical structure of the heart itself.The authors suggest that prolonged exposure to light at night could contribute to processes that place additional stress on the cardiovascular system.However, more research is needed to understand the biological mechanisms behind the association and determine whether reducing nighttime light actually lowers cardiovascular risk.Could making your bedroom darker help?The practical implications of the findings are relatively simple.Researchers and accompanying editorial experts suggest paying more attention to the sleeping environment, particularly for people who already have cardiovascular conditions.Keeping curtains or blinds closed can reduce outdoor light entering a bedroom. People may also choose dimmer bedside lighting and avoid unnecessary bright screens close to bedtime.Small indicator lights on electronic devices can also contribute to the amount of light in an otherwise dark room. Covering or switching off unnecessary LEDs may help create a darker sleep environment.The broader issue is light pollution, which has become increasingly common as cities expand and remain illuminated throughout the night.Street lighting is essential for safety, transport and public life, but researchers argue that lighting can be designed in ways that reduce unnecessary exposure. Shielded fixtures, warmer-spectrum lighting and systems that dim or activate only when needed are among the approaches being considered.For individuals, however, the simplest step may be inside the bedroom.A darker sleeping environment is unlikely to be a substitute for established measures such as controlling blood pressure, exercising, eating a balanced diet or avoiding tobacco. But if future research confirms that nighttime light contributes to cardiovascular problems, reducing unnecessary exposure could become another inexpensive part of heart-health advice.The researchers emphasise that their findings should be interpreted alongside the broader evidence rather than as proof that bedroom lighting directly causes heart disease.Still, the study offers a striking reminder that environmental factors affecting the heart may not always be obvious.The light that helps us see at night may be useful for modern life. But when it follows us into bed, researchers are increasingly asking whether darkness itself may be an important part of healthy sleep.Frequently asked questionsCan sleeping with the lights on affect the heart?The new study found an association between greater nighttime light exposure and changes in heart structure and function. It does not establish that sleeping with lights on directly causes heart disease.How much nighttime light did the study examine?Researchers compared participants exposed to more than three lux of nighttime light with people experiencing almost no light at night. Light exposure was measured using wrist-worn sensors over seven days.What heart changes were linked to nighttime light?Higher nighttime light exposure was associated with a thicker left ventricular wall, reduced flexibility of the heart muscle and changes involving other chambers, including the right ventricle and left atrium.How can I make my bedroom darker at night?Closing curtains or blinds, reducing unnecessary bedside lighting, switching off electronic devices and covering bright indicator LEDs can help reduce nighttime light exposure. A darker bedroom may also support a more consistent sleep environment.