By Sandee LaMotte, CNN
(CNN) — Even a small amount of light exposure during sleep may change the shape and function of heart muscles, increasing the risk of future heart disease, a new study found.
“Light pollution has emerged as a new risk factor for cardiovascular disease,” said senior study author Dr. Lu Qi, the HCA Regents Distinguished Chair and Professor and director of the Tulane University Obesity Research Center in New Orleans.
“Our findings, together with evidence from other studies, suggest that reduction of nighttime light exposure should be considered as one of the potential strategies for preventing heart disease by clinicians and policy makers,” Qi said in a statement.
For quality sleep, experts say the bedroom should ideally be a maximum of 1 lux, which is quite dark. A lux is a method of measuring illumination levels set by the International System of Units — 3 lux is the equivalent of moonlight or light bleeding under a closed bedroom door, according to experts.
In this study, researchers found the heart muscle walls of people exposed to more than 3 lux of light while asleep thickened in ways that reduced the efficiency of the heart’s pumping action.
The extent of damage rose as the bedroom grew brighter, according to the study. Examples of extra light might include the glow of a face-up smartphone, radiance from a digital alarm clock or light pollution from outside.
Thicker heart muscles may lead to heart failure
The study, published Wednesday in the European Heart Journal, analyzed the sleep of more than 11,000 people without cardiovascular disease who were part of the UK Biobank study, a biomedical database containing genetic and health data from 500,000 residents of the United Kingdom.
All study participants wore a wrist sensor for seven days to measure light levels during the night. Three years later, each person had a cardiovascular MRI to examine their heart structure and function.
Compared with people with no light exposure at night, sleeping in more than 3 lux thickened the walls of the left ventricle chamber of the heart, reducing the space inside. There was also a reduction in the heart muscle’s ability to flex during a heartbeat — an early indicator of heart dysfunction. Additional damage was also found in two other chambers.
The reshaping of heart muscle is called cardiac “remodeling,” said Dr. Andrew Freeman, director of cardiovascular prevention and wellness at National Jewish Health in Denver.
“We don’t want the heart muscle to get thick. We don’t want the heart muscle to get stiff. We don’t want the heart to get too big,” said Freeman, who was not involved in the study. “And all of these things seem to happen when exposed to excess light and potentially poor quality sleep.”
Between 24% and 49% of the impact on the heart was due to a shorter sleep duration of less than five or six hours, which is the “direct consequence of night light exposure,” Qi said. Other factors may account for the other 51% of risk.
“Night light exposure may affect other cardiovascular disease risk factors such as stress, biological changes,” Qi said in an email, referring to issues such as blood pressure and heart rate.
The study had limitations, said Karl Lawrence, a lecturer in the Institute of Pharmaceutical Science at King’s College London. For example, one week of testing is rather short to measure overall exposure. In addition, the use of a light sensor on the wrist “isn’t a good proxy for the dose received by the eye — this could be both over or under estimation.”
“To correlate this to heart scans three years later feels like it is somewhat of a stretch,” Lawrence said in an email. “Further studies would be needed to demonstrate causality.”
Blocking light at night
It’s relatively easy to block light in the bedroom, experts say. Place heavy drapes or inexpensive blackout curtains or shades on each window. Don’t forget to open them in the morning to bathe in the daylight that helps set the body’s circadian rhythm.
Turn off hallway lights at night. However, if light still creeps in under the door or the edges of windows, try light blocking foam or strips, which may be sold as draft stoppers or weatherproofing materials. If low levels of light persist, try a sleep mask.
Choose bedside reading light bulbs of warm white, deep amber or red that have low or near-zero effective color temperatures. If you need a night-light, keep it dim and at floor level and choose bulbs with reddish or brownish tones.
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