Exposure to light during sleep, including relatively low levels such as light from a phone screen or through a partially open door, has been linked to potentially harmful changes in the structure and function of the heart, according to a new study.
The research, published in the European Heart Journal, examined whether exposure to different levels of light at night was associated with changes in the heart several years later.
The study involved 11,071 people who were exposed to varying levels of nighttime light for one week. The levels ranged from near total darkness to light comparable with a phone screen at low brightness or light entering through a gap in a door.
Three years after the week of monitoring, participants underwent cardiovascular magnetic resonance scans. Researchers used the scans to assess the structure and function of their hearts.
Those exposed to the highest levels of nighttime light were more likely to show thickening of the walls of the heart’s left ventricle. The change reduced the space available inside the chamber. Researchers also identified signs that the heart had become less able to flex during a heartbeat, which can indicate early impairment in cardiac function.
Lu Qi of Tulane University in New Orleans, who led the research, said higher nighttime light exposure was associated with cardiac remodelling.
Cardiac remodelling refers to changes in the structure and function of the heart that can occur in response to prolonged stress or injury. Qi said the process can eventually weaken the heart and contribute to heart failure.
He said keeping bedrooms as dark as possible should be considered as one possible strategy for preventing cardiovascular disease.
The study’s findings were also discussed in an accompanying editorial by Professor Thomas Münzel of University Medical Center Mainz in Germany and colleagues. They argued that darkness should receive greater recognition as an important factor in cardiovascular health.
The researchers urged doctors to ask patients about their sleeping environments, including how dark their bedrooms are, whether they work night shifts and how much they use screens after sunset.
They suggested practical measures such as installing blackout curtains, choosing warmer-coloured bedside lighting and covering small standby lights on household electronics.
The editorial also highlighted the role of local authorities in reducing light pollution. Cities can use shielded lighting fixtures, warmer-spectrum bulbs and streetlights that dim or activate only when needed.
The authors said these measures could reduce unnecessary nighttime light exposure while also saving energy.
The researchers noted that the findings point to an association between nighttime light exposure and changes in heart structure and function, rather than proving that light exposure directly causes heart disease. Further research will be needed to understand the biological mechanisms involved and determine whether reducing nighttime light can lower cardiovascular risk.
