A recent study strengthens evidence that sleeping with ambient light increases cardiovascular risk. Greater exposure to light at night (LAN) while sleeping was linked to stress-related brain activity and arterial inflammation — processes that contribute to heart attacks and strokes. Over roughly a decade of follow-up, 17% of participants experienced a major cardiovascular event, and higher nighttime light exposure predicted greater long-term risk.
What the study did and found
The research followed 466 adults who had PET/CT brain scans at Massachusetts General Hospital between 2005 and 2008. Median age was 55; 43% were men, about 90% were white, and none had heart disease or cancer at the start. The CT scans provided anatomical detail while PET measured metabolic activity in the brain.
Investigators estimated outdoor lighting at each participant’s home using satellite data (Suomi NPP VIIRS Day/Night Band) combined with radiative transfer modeling to produce local ground-level sky brightness estimates (this approach excluded starlight and moonlight). They observed that higher LAN was associated with greater stress-linked brain activity and more arterial inflammation.
Key quantified risks: each standard-deviation increase in measured nighttime light was linked with a 35% higher chance of a heart attack over the next five years and a 22% higher risk over ten years. People living in neighborhoods with greater environmental or social stressors (for example, more traffic noise or lower income) faced even larger effects from LAN.
How light at night can harm sleep and the cardiovascular system
Researchers believe LAN disrupts neurophysiological systems that regulate metabolism, inflammation, and circadian rhythms. Experimental studies show measurable effects: sleeping in a typical lit bedroom (~100 lux) compared with very dim conditions (<3 lux) increased heart rate, reduced deep (slow-wave) sleep, and impaired insulin sensitivity. A meta-analysis of cross-sectional studies linked higher LAN exposure — particularly indoor light — with about a 22% greater risk of sleep problems.
Biologically, darkness stimulates melatonin production and supports the normal nightly drop in blood pressure. Persistent light exposure can delay melatonin onset, blunt the nighttime blood pressure decline, and shift morning hormone timing (for example, cortisol), all of which promote stress-related inflammation. That inflammation accelerates atherosclerosis in vessels serving the heart and brain. Short sleep duration (under six hours) is another independent risk factor for heart disease.
Practical ways to darken your sleep environment
– Install blackout curtains or shades; even inexpensive options such as tightly applied aluminum foil can block incoming light.
– Seal gaps around doors and windows; block light under doors with a towel or draft stopper.
– Use a well-fitting sleep mask to eliminate light at the eyes.
– Replace always-on nightlights with motion-activated models that light only when needed.
– Turn off or shield outdoor lights that shine into bedrooms when possible.
Bottom line
Sleeping in a darker environment — ideally complete darkness at the eyes — helps preserve restorative sleep physiology, supports normal circadian hormone patterns, and may reduce inflammation-related cardiovascular risk. Minimizing bedroom light is a simple, low-cost step that could contribute to long-term heart and brain health.