Somewhere between childhood nightlights and adulthood phone screens, darkness lost its place. The average bedroom today glows. Whether it’s a streetlamp slipping through the curtains, the steady glow of a television left on, or the pale blue beam of a charging phone, the human body registers those signals as instructions. And the consequences are not as harmless as many assume. 

For most of human history, sleep was governed by the rise and fall of the sun. Today, artificial light makes it easy to blur that boundary. What’s less obvious is how much that blur affects the body. Research in the past decade has shown that even dim light during sleep can interfere with how deeply you rest, how your metabolism functions, how your cardiovascular system responds the next day, and could even harm your heart health. 

Light vs. the Body’s Internal Clock 

We’re built on a cycle older than civilization: the human body runs on a schedule dictated by a master regulator known as the circadian rhythm. This internal clock, anchored by the brain’s suprachiasmatic nucleus (SCN), takes cues from sunlight to manage when we feel alert, when we feel tired, and even when organs carry out specific functions. For thousands of years, light meant daytime activity and darkness meant recovery. 

The problem comes when artificial light interferes with this system. Research has shown that exposure to even low levels of light at night can confuse the SCN, leading the body to misinterpret its surroundings. Essentially, the brain gets mixed signals: outside conditions say it’s time to sleep, but the light inside suggests it’s time to stay awake. The outcome is lighter, more fragmented sleep, leaving the body unprepared for the demands of the next day. 

Even with eyelids closed, the eyes remain sensitive to brightness. Closing your eyes doesn’t block out the effects. Eyelids still allow light to pass through, and the retina signals the brain to stay alert. Research in the Journal of Korean Medical Science showed that exposure to as little as 10 lux—the brightness of a dim nightlight—led to eye fatigue, increased eye pressure, and vision discomfort in sleepers. People who already spend long hours reading or working on screens, this compounds the strain. Instead of getting a break overnight, the eyes continue to respond to subtle stress, which may worsen discomfort over time.  

For context, a typical office is lit at around 500 lux. That means something as subtle as a hallway light slipping under the bedroom door is enough to interfere. Over time, this kind of low-grade strain contributes to ocular fatigue, making the eyes work harder not just during sleep but during waking hours too. 

How Does Sleeping with Lights on affect Your Quality of Sleep  

Interrupted sleep is rarely just about feeling groggy in the morning. The ripple effect goes deeper. Per Harvard Health, researchers have linked disrupted circadian rhythms to weight gain, higher blood sugar, and an increased risk of type 2 diabetes. In controlled studies, subjects exposed to night-time light developed signs of prediabetes and changes in leptin (the hormone that signals satiety). Another key player is melatonin. Light, especially in the blue spectrum, suppresses melatonin production. Lower melatonin isn’t only linked to insomnia, it has also been associated with increased risk of certain cancers, as melatonin may help play a role in slowing tumor growth, which makes its suppression more concerning. 

Certain Situations That Might Make Sleeping With the Lights On Actually Helpful 

Most of the time, it’s healthiest to sleep in complete darkness. But in certain situations, having a small light can actually be useful. For example, children or adults who feel anxious in complete darkness may sleep more soundly with a soft nightlight nearby. Similarly, Older adults may also benefit from a dim light to avoid tripping or falling if they need to get up during the night, reducing the risk of falls. In these cases, using a gentle, warm-colored light is recommended, it provides comfort and safety while minimizing the downsides of light exposure. 

Why the Color of Light Matters 

The type of light matters as much as the brightness. Blue and white lights common in LED bulbs, phones, and televisions—are particularly disruptive. It keeps the brain alert by suppressing melatonin more strongly than other wavelengths. White light, common in overhead fixtures and fluorescent bulbs, has a similar effect. The Centers for Disease Control and Prevention (CDC) notes that these wavelengths strongly suppress melatonin production, delaying the onset of sleep. 

By contrast, warmer colors like red, orange, and amber tones, have minimal impact on circadian rhythm. This is why many devices now include “night mode” settings, shifting displays from blue to amber hues after sunset. Similarly, swapping out a bright white bedside bulb for a dim red one can create the calming effect of firelight, helping the brain transition toward rest. 

Practical Ways to Improve Sleep 

Total darkness may be out of reach for many, but there are practical ways to limit exposure: 

• Blackout curtains or blinds help block intrusive streetlights.

• Sleep masks provide a portable solution for travel or shared spaces.

• Dimming indoor lights at least an hour before bed helps the brain ease into sleep mode.

• Device settings that reduce blue light exposure after sunset are effective, though avoiding screens entirely is better.

• Replacing bright white bulbs with red or amber lamps near bedtime can create a gentler transition to sleep. Even these small shifts can help restore balance to the circadian rhythm, allowing the body to work with its biology rather than against it.