Darkness in your bedroom may lower your heart attack risk

A new study presented at the American Heart Association Scientific Sessions 2025 found something straightforward: sleeping in a lit room activates stress pathways in your brain that inflame your arteries. Over ten years, participants exposed to more nighttime light were 35% more likely to have a heart attack and 22% more likely to have a stroke.

The research tracked 466 adults from Massachusetts General Hospital using satellite data to measure light exposure around their homes. Researchers used NASA imaging to estimate how much artificial light was reaching each bedroom, then followed participants for a decade. The pattern held steady across the group: more light at night, higher cardiovascular risk. People living with additional stressors—traffic noise, lower neighborhood income—faced even steeper odds.

Why Your Body Needs Darkness

When light reaches your eyes at night, it disrupts the signals that tell your body to produce melatonin and lower blood pressure. Your cardiovascular system is supposed to shift into recovery mode after dark. Without that darkness cue, your heart stays in a subtly stressed state all night long.

Cardiologist Jayne Morgan explains the mechanism plainly: "Inflammation is a result of stress, and inflammation is a primary driver of atherosclerosis and therefore heart disease and stroke." Even modest indoor lighting—around 100 lux, roughly what a bedside lamp produces—has been shown to reduce deep sleep, raise heart rate, and impair insulin sensitivity.

This isn't about blue light from your phone, though that matters too. It's about the ambient glow in your room. A 2022 meta-analysis found that people exposed to higher levels of artificial light at night had a 22% greater risk of sleep problems. Earlier research in older adults linked nighttime light directly to atherosclerosis, the thickening of artery walls that leads to heart attacks and strokes.

Making Your Room Actually Dark

The fix doesn't require expensive equipment. Blackout curtains block streetlight and dawn. A rolled towel under your bedroom door stops hallway light from creeping in. A fitted sleep mask works if you travel or share a space. Motion-sensing nightlights replace always-on lights—they turn on only when you need them.

The study itself hasn't been peer-reviewed yet, but sleep scientists say the findings align with existing evidence. Jonathan Cedernaes, a sleep researcher at Uppsala University in Sweden, notes that both animal and human studies have shown artificial light at night disrupts multiple systems in your body.

The takeaway is simple: if you're serious about heart health, darkness matters as much as exercise or diet. Your bedroom should be as close to pitch black as you can make it.