Sleeping with a night light, leaving the television on or allowing streetlights to shine into the bedroom may do more than affect sleep quality — prolonged exposure to light at night could also be linked to changes in heart structure and function, according to new research.
Nutritional and functional medicine doctor Liu Bor-ren highlighted findings from a study that analysed data from the UK Biobank, involving 11,071 participants who wore light-sensing devices continuously for seven days to record their actual exposure to light at night.
Researchers then used cardiac magnetic resonance imaging (MRI) to assess the participants’ heart structure and function.
The study found that compared with people exposed to little or no noticeable light at night, those with higher levels of nighttime light exposure had an approximately 2.4% increase in left ventricular muscle mass and a 1.5% increase in average ventricular wall thickness.
Their cardiac contractile function also declined by about 1.9%, while unfavourable structural and functional changes were also observed in the right ventricle and left atrium.
The researchers found that the greater the exposure to light at night, the more pronounced the associated changes appeared to be.
In a further analysis of more than 70,000 people followed for around eight to 10 years, researchers found that those with the highest levels of nighttime light exposure also had higher risks of developing heart failure and atrial fibrillation, as well as experiencing heart attacks, strokes and cardiovascular death.
Liu said the human body relies on a finely regulated circadian rhythm, and continued exposure to light at night may interfere with melatonin production, sleep, the autonomic nervous system and metabolism.
The research also suggested that shorter sleep duration could explain around 24% to 49% of the observed associations, meaning nighttime light may first disrupt sleep before indirectly affecting cardiovascular health.
However, the findings come from an observational study and do not prove that sleeping with a night light directly causes heart disease.
Liu said there was therefore no need for people to panic over occasionally sleeping with a light on, but recommended keeping bedrooms as dark as possible at night.
He suggested using blackout curtains to block outdoor light and avoiding leaving phones, tablets or televisions switched on throughout the night.
For those who need a night light, he recommended choosing one with low brightness and positioning it so that the light does not shine directly into the eyes.
