Too Much Light At Night May Physically Change Your Heart, Study Finds

We spend a lot of time trying to perfect our sleep environments. We turn down the thermostat, invest in better mattresses, put our phones away earlier, and maybe even track exactly how long we spend in deep sleep.

But there's one part of the bedroom that's surprisingly easy to overlook: how dark it actually is.

Maybe there's a streetlight coming through the curtains, a glowing alarm clock next to your bed, or enough ambient light from outside that you can still make out everything in the room. It may not seem bright enough to matter, especially if you're still able to fall asleep.

In a new study, researchers analyzed data from more than 11,000 adults to see whether exposure to light at night was associated with changes in the heart's structure and function.

Rather than asking people how dark their bedrooms were, participants wore wrist sensors for seven days that objectively measured their exposure to light overnight. About three years later, they underwent cardiac MRI scans, giving researchers a detailed look at the size, shape, and function of different chambers of the heart.

The researchers then compared those scans with participants' nighttime light exposure, while accounting for other factors that could influence cardiovascular health.

Compared with people who were exposed to almost no light overnight, participants exposed to more than three lux showed several differences in the structure and function of their hearts.

For context, three lux is still fairly dim. You don't need to be sleeping with an overhead light on to reach that level. Ambient light from outside, a television, or other light sources in the bedroom can all contribute to nighttime exposure.

Among those with greater light exposure, researchers saw thickening of the wall of the left ventricle, the heart's main pumping chamber, along with less space inside the chamber itself. They also found signs that the heart muscle wasn't relaxing and flexing as effectively.

Researchers describe this type of structural change as cardiac remodeling. The heart can remodel in response to ongoing stress, changing its size, shape, or thickness to adapt. That response isn't necessarily harmful in every context, but over time, certain patterns of remodeling can leave the heart stiffer and less efficient and are associated with conditions such as heart failure.

The connection becomes easier to understand when you think about light as biological information rather than simply something that helps us see.

Our bodies use changes between light and darkness to regulate the circadian clock, which helps coordinate sleep, hormone release, metabolism, blood pressure, heart rate, and many other processes across a 24-hour cycle.

Normally, nighttime darkness tells the body that it's time to shift into its overnight state. Light exposure can interfere with that signal, potentially suppressing melatonin and disrupting normal circadian rhythms. Researchers suspect that repeated disruption may influence the autonomic nervous system, blood pressure, inflammation, and metabolic function, all of which can affect the cardiovascular system over time.

We can't control every source of light pollution, particularly if you live in a city or near a busy street. But the bedroom itself is usually much easier to modify.

Blackout curtains or shades can make a big difference if outdoor light comes through your windows. Cover or turn off unnecessary LEDs from chargers, televisions, alarm clocks, and other electronics, and consider an eye mask if getting the room completely dark isn't realistic.

It's also worth thinking about the hour or two before bed. Dimming overhead lights and opting for warmer, softer lighting in the evening can help reinforce the transition from daytime to nighttime rather than flooding your eyes with bright light right up until you get into bed.