Scientists have reported on the connection of night light with changes in the heart

Exposure to light at night is associated with changes in the structure of the heart and early signs of deterioration of its function. This conclusion was reached by researchers who studied the data of more than 11 thousand participants of the British project UK Biobank. In people with a higher nighttime light load, thickening of the walls of the left ventricle and less favorable performance of the heart muscle were found. How scientists explain this connection and what they propose to change in bedrooms and cities is in the "Izvestia" material.

How scientists studied night lighting

The work, led by Lu Qi, a professor at Tulane University in New Orleans, was published on September 9 in the European Heart Journal. The researchers were looking for a possible explanation for the link between night light and cardiovascular disease: previous observations indicated an increased risk, but there was insufficient data on the condition of the heart muscle itself.

The study included 11,071 people. For seven days, participants wore a device with a light sensor on their wrist. About three years later, they underwent a magnetic resonance imaging of the heart, which allowed them to assess its structure and function. The researchers then compared the survey results with previously collected light data.

The night period was determined individually by the five hours of least activity. The analysis took into account the effect of light brighter than three lux. This threshold should not be perceived as an established security boundary. The sensors allowed us to evaluate the lighting that people encountered in their daily lives. At the same time, the work does not give grounds to attribute the detected differences to a separate source: a night light, a TV screen, or a street lamp.

What changes were found in the heart

Participants with high nighttime light exposure were found to have thicker walls of the left ventricle, one of the main chambers of the heart. At the same time, there were signs of deterioration in the mechanics of the heart muscle: during contraction, it changed shape less effectively.

According to the article, in the group with high exposure, compared with the group without recorded exposure to light above three lux, the indexed mass of the left ventricle was on average 2.4% greater, the average thickness of its walls was 1.5% greater, and the rate of myocardial contraction was 1.9% lower.

The changes also affected the right ventricle and the left atrium. Thus, the discovered connection concerned several chambers of the heart. The authors consider the results as signs of unfavorable remodeling — restructuring of the structure and function of the organ.

According to his explanation, such a restructuring usually occurs in response to chronic stress or damage. Initially, it helps the body to adapt, but over time it can be accompanied by a weakening of the heart and the development of heart failure.

However, the MRI results do not mean that all people who sleep in the light already have the disease. We are talking about average differences between groups and early changes in individual indicators, the value of which must be assessed together with other health data.

Why light at night can affect the body

One of the possible mechanisms is related to circadian rhythms — daily changes in physiological processes. The alternation of light and dark helps the body to coordinate sleep, wakefulness and the work of internal organs with the time of day.

As the US National Institute of General Medical Sciences explains, almost all tissues and organs have their own circadian rhythms. They are coordinated by the central biological clock system in the brain. She receives information about light through her eyes and participates in controlling the production of melatonin, a hormone associated with the onset of drowsiness.

Evening and night light can interfere with this setting. In particular, light from electronic devices can disrupt the consistency of the biological clock with the external day and night cycle. Prolonged lack of sleep and persistent circadian rhythm disturbances are associated with metabolic, heart, and blood pressure problems.

In the new study, a reduction in sleep duration statistically explained from 24 to 49% of a number of discovered links between night lighting and heart indicators. Such an assessment indicates a possible role of sleep, but by itself does not prove the mechanism of action.

For cardiovascular prevention, sleep already has an independent meaning. The American Heart Association has included it in the list of eight essential components of heart health, along with physical activity, nutrition, and risk factor control. The organization recommends sleeping seven to nine hours a night for most adults.

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What experiments with light during sleep have shown

Physiological changes in night light have previously been observed in the laboratory. The Northwestern University study, published in 2022 in PNAS, involved 20 healthy young adults. The participants were divided into two groups. One slept for two nights in dim lighting below three lux. The other spent the first night in the same conditions, and the second — with the room light turned on with illumination of 100 lux.

After a night with indoor lighting, the researchers found a higher heart rate during sleep and a decrease in heart rate variability. These changes corresponded to increased activity of the sympathetic nervous system. In the morning, participants also had higher rates of insulin resistance, a condition in which the body responds less well to the effects of insulin.

At the same time, the level of melatonin did not significantly differ between the conditions. Therefore, the authors discussed increased sympathetic activity as one of the possible ways light affects metabolism.

The experiment showed that changes in physiological parameters can occur even after one night in room lighting. However, a small sample and a short duration of follow-up limit the conclusions: this work does not establish whether such a regime leads to a chronic disease, and does not determine the risk from a weak night light.

How night light is associated with the risk of diseases

Longer-term follow-up also indicates an association of night lighting with cardiovascular risk. In October 2025, the JAMA Network Open published a study involving 88,905 people over the age of 40 from the UK Biobank. Their light load was measured with wearable sensors, and subsequent diagnoses were tracked using medical data.

Compared with the participants from the half of the sample with the darkest nights, people from 10% with the brightest night lighting had an estimated relative risk of heart failure by 56%, myocardial infarction by 47%, and atrial fibrillation by 32%. The links were maintained with additional adjustments for a number of risk factors.

These percentages describe the relative differences between the groups. They do not mean that 56% of people with high light exposure have heart failure, and they do not allow us to calculate personal risk based on the brightness of the lamp in the bedroom.

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How to reduce the light load in the bedroom

The authors of the accompanying editorial suggest that doctors pay more attention to sleep conditions. Thomas Munzel, professor at the University Medical Center of Mainz, and his colleagues find it especially useful to discuss them with patients who already have heart failure or atrial fibrillation.

Among the proposed measures are blackout curtains, warm—toned bedside lighting and the closure of small luminous indicators of household appliances. Such changes help to reduce unnecessary lighting during rest hours. However, the new work did not test exactly how each of these measures affects the risk of disease.

In addition, experts advise keeping the bedroom as dark as possible, reducing exposure to light in the evening and getting enough light during the day, mostly natural. The recommendations use a special indicator that takes into account the effect of the spectrum on the photosensitive system of the eye — melanopic equivalent daylight. It cannot be directly equated with the ordinary lux measured by the wrist sensor in the new study.

If lighting is necessary for safe movement at night, experts suggest using a weak source closer to the floor and further away from the sleeper's face. Blackout curtains or a sleep mask can help in case of external illumination. Recommendations for reducing light should take into account safety and real human needs.

Why does night lighting affect entire cities?

The light load also depends on what is outside the window: street lights, building lights, and other sources that work all night. Therefore, the authors of the editorial consider the problem at the level of the urban environment.

In their opinion, municipalities can reduce excessive illumination by using fixtures that direct light to where it is needed, warm spectrum lamps, brightness control and motion sensors. Such solutions are already available and allow you to combine the lighting of public spaces with a darker night environment.

The scale of the changes is illustrated by a paper published in Science in 2023. According to the observations of volunteers on the visibility of stars in 2011-2022, the changes corresponded to an increase in the brightness of the night sky by about 9.6% per year. This was reported by the American Association for the Advancement of Science. However, the brightness of the sky and the amount of light reaching a person's eyes in the bedroom are different indicators.

Qi believes that reducing nighttime light exposure deserves consideration in the prevention of heart disease along with other measures.