Sleeping in a room with even a small amount of light can alter the shape and function of heart muscles and increase the risk of future heart disease.
According to a new study published on Wednesday in the European Heart Journal, light pollution is emerging as a new risk factor for cardiovascular disease. The European Heart Journal is a peer-reviewed medical publication produced on behalf of the European Society of Cardiology.
The study analysed the sleep patterns of more than 11,000 people without existing cardiovascular disease. The participants were drawn from the UK Biobank, a long-term biomedical database containing the genetic and health information of 500,000 residents across the United Kingdom.
Participants wore wrist sensors for seven days to record their light exposure during the night. Three years later, each person underwent a cardiovascular magnetic resonance imaging scan to assess the structure and function of their heart.
Damage from minimal light
Experts recommend that a bedroom should have a maximum of one lux for a healthy night of sleep. Lux is a standardised unit of illumination used in the International System of Units. According to researchers, three lux is roughly equivalent to the brightness of a moonlit night or the glow coming from under a closed bedroom door.
The researchers found that individuals exposed to more than three lux of light during sleep experienced thickening in the walls of their heart muscles. This thickening reduced the efficiency of the heart as it pumped blood.
Compared to people without nighttime light exposure, those exposed to more than three lux had thicker left ventricle walls, which reduced the internal space of the chamber. Their heart muscles also showed a reduced ability to contract during each heartbeat, which is an early indicator of cardiac dysfunction. The scans revealed additional damage in two other cardiac chambers.
The study showed that the extent of the damage increased as bedrooms became brighter. Common sources of this extra light include a smartphone placed with its screen facing upwards, the glow of a digital alarm clock, and external street lighting.
Impact of sleep deprivation
Lu Qi, the senior author of the study and the director of the Obesity Research Center at Tulane University in New Orleans, said the findings suggest that doctors and policy makers should view the reduction of nighttime light exposure as a potential strategy to prevent heart disease. Tulane University is a private research institution in Louisiana.
Qi said that between 24 percent and 49 percent of the impact on the heart could be attributed to participants sleeping for fewer than five or six hours. He described this reduced sleep duration as a direct consequence of exposure to light at night. The remaining risk could be driven by other factors, with Qi noting in an email that nighttime light exposure can influence issues like stress, blood pressure and heart rate.
Andrew Freeman, the director of cardiovascular prevention and wellness at National Jewish Health in Denver, said this process is known as cardiac remodelling. National Jewish Health is a leading respiratory and immunological hospital in Colorado. Freeman, who was not involved in the research, said doctors do not want the heart muscle to thicken, stiffen or become too large, and that all of these outcomes appear to result from excessive light exposure and poor sleep quality.
Limitations of the research
Karl Lawrence, a professor at the Institute of Pharmaceutical Science at King's College London, noted several limitations in the research. King's College London is a public research university and one of the oldest university level institutions in England.
Lawrence said in an email that a one week testing period was too short to measure total light exposure accurately. He also argued that a wrist sensor was a poor proxy for the amount of light actually reaching a person's eyes, meaning the measurements could result in significant overestimations or underestimations. He added that attempting to correlate these one week measurements with cardiac exams conducted three years later seemed somewhat forced, and that more studies would be required to prove a direct causal link.
Blocking light sources
Experts advise that blocking out light in a bedroom is relatively simple. They recommend installing heavy curtains or inexpensive blackout blinds, though these should be opened in the morning to allow natural light to help regulate the circadian rhythm of the body.
Hallway lights should be turned off at night. If light continues to enter from under a door or through window frames, experts suggest using foam strips or light blocking tapes designed to exclude draughts. A sleep mask can also be worn if low level light persists.
For bedside reading, experts recommend using warm white, dark amber or red bulbs with low or near zero effective colour temperatures. Any night lights should be kept dim, placed at floor level, and fitted with bulbs in reddish or brownish tones.
