High on Health: That Nightlight May Be Damaging Your Heart

We all know sleep is important to our health, especially the amount we get. But now, a new study suggests there’s another factor in our nighttime rituals that might need to be taken into consideration. A study published in the European Heart Journal on September 9, proposes that the amount of light in your bedroom may cause heart damage.

Light and Heart Health

Everyone has their own recipe for the perfect night’s sleep. Some need the room cold enough to hang meat, while others pile on blankets like they’re preparing for an Arctic expedition. There are white-noise devotees, fan addicts, TV sleepers, and people who can’t drift off without scrolling through their phones for “just five more minutes.” Then there’s light. Some of us want a bedroom so dark we couldn’t find the door if the house were on fire, while others keep a night light glowing, the hallway light on, the TV flickering, or the curtains cracked for a little reassurance. Turns out, our hearts may have an opinion on that last one. New research suggests that exposure to light while we sleep, even low light, is associated with changes in the heart and a higher risk of cardiovascular problems.

“Light pollution has emerged as a new risk factor for cardiovascular disease,” said senior study author Dr. Lu Qi. “Our findings, together with evidence from other studies, suggest that reduction of nighttime light exposure should be considered as one of the potential strategies for preventing heart disease by clinicians and policy makers.”

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Researchers used data from the UK Biobank. For the main part of the study, they studied 11,071 adults who did not have cardiovascular disease at the start of the study. The average age was about 61, and 56.6% were women.

Participants wore a wrist device with a light sensor for seven consecutive days between 2013 and 2015, which allowed researchers to measure how much light people were actually exposed to during their normal nighttime sleeping, instead of just estimating outdoor light based on where they lived. Three years later, an MRI (magnetic resonance imaging) was performed to see if there were any changes.

The study revealed that the people with the highest nighttime light exposure had several differences. Those with a high light exposure had 2.4% greater left ventricular mass, 1.5% greater average heart wall thickness, and a 1.9% lower myocardial contraction fraction, which is a measure of how effectively the heart muscle contracts.

Researchers adjusted for several factors, including age, sex, activity, diet, obesity, and other health problems, and found that those exposed to high light exposure above three lux had:

• 29% higher risk of heart failure
• 15% higher risk of atrial fibrillation
• 24% higher risk of heart attack
• 33% higher risk of stroke
• 26% higher risk of dying from cardiovascular disease

The researchers concluded: “Greater nighttime light exposure was associated with adverse cardiac remodeling and subclinical functional decline across multiple chambers, statistically mediated by shorter sleep duration. These findings provide potential mechanistic insights into how nighttime light exposure may elevate CVD risk and highlight dark nights and adequate sleep for cardiovascular health.”

Levels of Light Exposure

How much light is too much? The study focused on exposure to more than three lux, but measuring that is not so straightforward. You can't reliably say that a particular lamp, TV, or phone always equals a certain number of lux. Lux is the amount of light reaching a particular surface or sensor, so distance, brightness settings, direction, room size, shades, and other factors all matter. So, 3 lux isn’t pitch black, but more like what you’d expect in a room where you can barely see your surroundings but aren’t sitting in total darkness.

A night light can exceed three lux if it’s bright or close to you, whereas a faint one across the room might not.

Do you like to have background noise and light while you sleep, such as leaving a TV playing? The light exposure rate will vary greatly depending on several factors, including the screen’s brightness, size, and distance from your bed. A bedside lamp, on the other hand, would ordinarily be well above three lux while it’s on. Typical indoor room illumination used in sleep research is much brighter than the dim light conditions, and one laboratory study used 50 lux as the bedroom-intensity light.

You may think having your phone or tablet nearby is a good thing, but they expose your eyes to significant light even if they don’t illuminate the room. A controlled study found that evening use suppressed melatonin – the hormone that helps us sleep – by about 55% compared to reading a printed book.

A hallway or bathroom light sneaking in from under the bedroom door might not be as innocent as you think. It has the ability to push above 3 lux, depending on brightness and distance. Street and porch lights or other outdoor lighting coming through the window could potentially do it as well.

So, add “how dark is my bedroom?” to the ever-growing list of things apparently worth considering before climbing under the covers. We already have to worry about getting enough sleep, keeping the room cool, putting down the phone, and resisting the urge to watch just one more episode of a favorite show. Now our hearts may prefer that we turn out the lights, too. Of course, this is an observational study, so there’s no need to start covering every glowing clock and charging cable with electrical tape or stumbling through the bedroom in total darkness. Still, if the TV has been doubling as your night-light and the streetlamp outside makes your room look like sunrise at 2 a.m., blackout curtains might not be the worst addition to your sleep routine. Apparently, when mom said, “lights out,” she may have been offering health advice without even knowing it.