A new study adds to evidence that sleeping in anything less than complete darkness may harm the cardiovascular system. Higher levels of light exposure while sleeping were linked to stress-related brain activity that can cause arterial inflammation, raising the risk of heart attack and stroke.

Study findings
– Researchers followed 466 adults who had PET/CT brain scans at Massachusetts General Hospital between 2005 and 2008. Median age was 55; 43% were male; nearly 90% were white. None had heart disease or cancer at baseline.
– Ground-level night brightness at participants’ homes was estimated using satellite (VIIRS Day/Night Band) data with radiative transfer modeling (excluding natural starlight and moonlight).
– After 10 years, 17% of participants had developed major cardiovascular events. Each standard deviation increase in nighttime light exposure was associated with a 35% higher likelihood of a heart attack over five years and a 22% higher risk over 10 years.
– People living in areas with higher environmental or social stressors (for example, traffic noise and lower neighborhood income) showed even greater risk from light-at-night (LAN).

How light at night may affect heart health
– LAN can disrupt neurophysiological systems, affecting metabolism and inflammation, experts say. Inflammation driven by stress contributes to atherosclerosis, which underlies heart disease and stroke.
– Darkness promotes melatonin production and the normal nighttime drop in blood pressure; light at night can delay these healthy processes and shift morning hormone timing (including cortisol), which may harm cardiovascular health.
– Blue light from screens is a known disruptor of sleep, but ambient indoor or outdoor light also impairs sleep quality and can increase physiological stress.

Supporting evidence
– A 2022 experimental study found that sleeping in a typical bedroom light level (~100 lux) versus very dim (<3 lux) increased heart rate, reduced deep sleep, and impaired insulin sensitivity.
– A 2022 meta-analysis of seven cross-sectional studies reported that greater LAN exposure, especially indoors, was associated with a 22% greater risk of sleep problems.
– Other cohort data have linked typical bedroom light levels prospectively to higher risk of cardiovascular outcomes such as myocardial infarction.

Practical tips for darker sleep
– Install blackout curtains or shades; cheaper alternatives like tightly applied aluminum foil can reduce window light.
– Reduce light leakage from doors by turning off hall lights or blocking gaps with a rolled towel.
– Use a well-fitting eye mask if comfortable.
– Replace always-on nightlights with motion-sensing lights that activate only when needed.

Summary
Evidence suggests that darker sleeping environments — ideally near total darkness — support better sleep physiology and may lower long-term cardiovascular risk by reducing stress-related inflammation and preserving normal nocturnal hormone and blood pressure patterns.

A new study adds to evidence that sleeping with ambient light raises cardiovascular risk. Higher exposure to light at night (LAN) during sleep was linked with stress-related brain activity that can cause arterial inflammation — a driver of heart attack and stroke. Over a decade, 17% of the study participants developed major cardiovascular events, and greater nighttime light was associated with higher long-term risk.

Study design and findings
The study examined 466 adults who underwent PET/CT brain scans at Massachusetts General Hospital from 2005 to 2008. Median age was 55; 43% were male, about 90% were white, and none had heart disease or cancer at baseline. CT provided anatomy and PET measured brain metabolic activity.

Researchers estimated lighting conditions at participants’ homes using the Suomi NPP satellite’s Visible Infrared Imaging Radiometer Suite Day/Night Band (VIIRS DNB) with radiative transfer modeling to generate localized ground-level sky brightness (this modeling excluded starlight and moonlight). They found that as LAN exposure increased, so did stress-linked brain activity and arterial inflammation.

Quantified risks: each standard-deviation increase in measured light was associated with a 35% higher likelihood of a heart attack over the next five years and a 22% increased risk over ten years. Those living in areas with higher environmental or social stressors (for example, more traffic noise or lower neighborhood income) were at even greater risk from LAN.

How light affects sleep and the heart
Experts say LAN can disrupt neurophysiological systems that regulate metabolism and inflammation. In experimental work, sleeping in typical bedroom light (~100 lux) versus very dim light (<3 lux) raised heart rate, reduced deep sleep, and impaired insulin sensitivity. A meta-analysis of cross-sectional studies found that higher LAN exposure, especially indoors, increased the risk of sleep problems by about 22%.

Inflammation from stress promotes atherosclerosis in both heart and brain vessels. Darkness cues melatonin production and supports the nighttime drop in blood pressure; persistent light exposure delays these restorative processes and can shift morning hormone timing (for example, cortisol). Short sleep duration (under six hours) independently raises heart disease risk.

Practical steps to darken your sleep environment
– Cover windows tightly with blackout curtains or shades; lower-cost options like aluminum foil can block light.
– Close gaps under or around bedroom doors; turn off lights outside or use a towel to block the gap.
– Consider a well-fitting sleep mask to block light at the eyes.
– Replace always-on nightlights with motion-sensing models that only activate when needed.

Bottom line: darker sleeping environments — ideally complete darkness — support better sleep physiology and may lower inflammation-related cardiovascular risk.