Blackout Blinds vs Curtains: Which Blocks Light Better

Last updated: February 2026

TL;DR: Traditional blackout curtains block 85–99% of light, but research from shows even moderate light exposure during sleep increases heart rate and insulin resistance after just one night. The gaps at curtain edges, where fabric meets the wall, allow light leak that can suppress melatonin production by over 50%. True blackout requires shades with sealed track technology that eliminate these gaps entirely. For optimal sleep quality and metabolic health, complete darkness is not a luxury but a biological necessity. Northwestern University

You bought "blackout" curtains. You hung them exactly as the instructions said. Yet every morning, light still creeps through the edges, waking you before your alarm. Those glowing strips along the sides mock your investment in better sleep.

You are not imagining the problem. And it is not your fault.

The issue is physics. Traditional curtains, no matter how dark the fabric, leave gaps where they meet walls, floors, and window frames. Those gaps might seem small, but research shows they create a problem far bigger than most people realize.

Here is what the science reveals: your brain does not need much light to disrupt your sleep. And the difference between "almost dark" and "completely dark" could be affecting your health in ways you have never considered.

The Real Problem With "Almost Dark"

Most people assume that blocking most light is good enough. After all, what difference could a sliver of light along the edge of a curtain really make?

According to research published in the Proceedings of the National Academy of Sciences, quite a lot.

Dr. Phyllis Zee and her team at Northwestern University studied what happens when people sleep with even moderate light exposure of around 100 lux, roughly the amount you might experience from a streetlight filtering through curtain gaps. The results were striking.

After just one night, participants who slept with light exposure showed elevated heart rates throughout the night and increased insulin resistance the following morning. Their nervous systems remained in a heightened state of alertness, even while asleep. The participants themselves reported sleeping fine, completely unaware their bodies were responding to the light.

"But the brain senses it," explained Dr. Daniela Grimaldi, a researcher on the study. "It acts like the brain of somebody whose sleep is light and fragmented."

This matters because up to 40% of people sleep with some form of light in their bedroom, whether from a lamp, TV, or light leaking around window treatments.

How Light Disrupts Your Sleep Hormones

Your body relies on darkness to trigger melatonin production, the hormone that regulates your sleep-wake cycle and signals your body to prepare for rest.

Research from Brigham and Women's Hospital and Harvard Medical School found that exposure to ordinary room light before bedtime suppressed melatonin in 99% of study participants. Light exposure also shortened melatonin duration by approximately 90 minutes and suppressed melatonin levels by more than 50% in 85% of trials.

For children, the effects are even more pronounced. A University of Colorado Boulder study found that preschoolers exposed to light in the hour before bedtime experienced melatonin suppression ranging from 70 to 99%, regardless of how dim the light was. Even 50 minutes after the light was turned off, melatonin levels had not recovered in most children tested.

"We were very surprised to find high melatonin suppression across all intensities of light, even dim ones," said lead researcher Dr. Lauren Hartstein.

This research points to a clear conclusion: when it comes to sleep, partial darkness is not the same as complete darkness.

Why Traditional Blackout Curtains Fall Short

Blackout curtains are designed with light-blocking fabric, often featuring multiple layers or specialized coatings that prevent light transmission through the material itself. The fabric typically blocks between 85 and 99% of light.

The problem is not the fabric. The problem is everything around it.

The Gap Problem

Traditional curtains hang from a rod, creating natural gaps:

• Side gaps: Where the curtain meets the wall, light enters from the sides
• Top gaps: The space between the rod and the window frame allows light above
• Bottom gaps: Unless curtains pool heavily on the floor, light enters beneath
• Center gaps: Where two curtain panels meet, light sneaks through

These gaps are the main reason standard drapes struggle to achieve 100% darkness. You can read more about the physics of why blackout curtains fail here.

Even when curtains are installed with wide rods extending past the window frame, light finds its way through. The fabric sways with air movement, creating momentary gaps. The weight of heavier blackout fabric can pull curtains inward, widening side gaps.

The Measurement Challenge

To truly block light with curtains, you need:

• Rods extending 6 to 12 inches beyond the window frame on each side
• Curtains mounted several inches above the frame
• Fabric that pools on the floor or attaches to the wall
• Overlap hardware or magnetic edges to seal center gaps

Even with all these modifications, light still penetrates. The physics of hanging fabric from a rod simply cannot create a complete seal against a flat wall surface.

How Blackout Blinds Work Differently

Unlike curtains that hang freely, blackout blinds mount directly to the window frame. This closer fit reduces the gap problem but does not eliminate it entirely.

Standard roller blinds sit within or just outside the window frame, leaving small gaps at the edges where the shade does not quite meet the wall. Some light escapes around these perimeter gaps.

The solution to the gap problem requires a different approach altogether: sealed track technology.

What Sealed Track Technology Does

Sealed track systems guide the edges of the shade through channels mounted to the wall. These channels eliminate the side gaps entirely. The shade travels up and down within the tracks, maintaining constant contact with the wall surface.

Combined with a top cassette that houses the rolled shade and a bottom bar that seals against the window sill, this approach creates a complete barrier against light intrusion. No air movement, no fabric sway, no gaps.

This is the difference between blocking most light and blocking all light.

This seal also creates a thermal barrier. For a deeper look at energy efficiency, check out our guide on Do Blackout Curtains Keep Heat Out?.

Who Needs Complete Darkness Most

While everyone benefits from sleeping in darkness, certain groups are particularly affected by light exposure:

Shift workers: People who sleep during daylight hours face constant light intrusion. Standard window treatments rarely block enough light for quality daytime sleep. Research on night shift workers shows that controlling light exposure is critical for maintaining healthy melatonin cycles.

Children and infants: Young children show heightened sensitivity to light, with melatonin suppression occurring at much lower light levels than adults. Creating a completely dark nursery supports better sleep patterns and longer sleep duration. If you are setting up a room for a baby, check out our guide on how to make a nursery pitch black for step-by-step tips.

Light-sensitive individuals: People with migraines, certain neurological conditions, or general light sensitivity often find that even small amounts of light disruption affects their sleep quality and next-day symptoms.

Home theater enthusiasts: Ambient light degrades picture quality, particularly for projectors and OLED displays. Complete light control transforms the viewing experience.

Health-conscious sleepers: Anyone prioritizing metabolic health, cardiovascular function, or circadian rhythm optimization benefits from eliminating nighttime light exposure.

The Long-Term Health Connection

The consequences of sleeping with light extend beyond feeling groggy the next morning.

A Northwestern University study of over 500 older adults found that those exposed to any light while sleeping were significantly more likely to be obese, have high blood pressure, and have diabetes compared to those who slept in darkness.

What surprised researchers most: fewer than half of participants consistently experienced a five-hour period of complete darkness each day.

A larger study tracking nearly 90,000 people using fitness monitor light-exposure data found that low ambient light during sleep was associated with higher risk of heart failure and other cardiovascular problems over approximately 10 years.

"Electric lighting is totally aberrant to our biology," notes researcher Angus Burns. "It's brand new, essentially, on the evolutionary scale, that we have light at night in this way."

The human body evolved to sleep in complete darkness. Modern lighting has changed our environment faster than our biology can adapt.

Making the Right Choice for Your Situation

Consider blackout curtains if:

• Budget is your primary constraint
• You want a soft, decorative look
• You accept that some light leak is inevitable
• Your sleep environment has minimal external light sources

If you decide that curtains are the right choice for your space, we have tested the top market contenders to help you find the most effective ones. Read our full review of the Best Blackout Curtains of 2026.

Consider sealed track blackout shades if:

• You need complete darkness for sleep quality
• You or family members are shift workers
• You have children who need optimal sleep environments
• You are concerned about the health effects of light exposure
• You want a home theater with zero ambient light
• You value long-term investment over upfront savings
• ⁠⁠You want to use tax-advantaged funds (check if your blackout shades are HSA/FSA eligible).
  

What 100% Darkness Actually Looks Like

UBlockout's Ultimate Blackout Shade represents the current standard for complete light elimination. The patented sealed track technology guides shade edges through wall-mounted channels, eliminating the gaps that plague every other window treatment option.

Over 600 customers have verified what the engineering promises: complete darkness, not 95%, not 99%, but true zero-light conditions.

The Ultimate Blackout Shade includes:

• Sealed side tracks that eliminate edge gaps
• Top cassette housing that blocks light above
• Weighted bottom bar that seals against the sill
• Optional motorization for daily convenience
• Smart home compatibility for automated schedules
• Cordless design for child safety

For those who prefer the aesthetic of flowing curtains, the Ultimate Blackout Shade works as an underlayer. You get complete light control from the shade with whatever decorative treatment you prefer in front.

Key Takeaways

• Traditional blackout curtains block 85–99% of light through the fabric, but gaps at edges allow significant light leak that can suppress melatonin by over 50%
• Northwestern University research shows even one night of moderate light exposure during sleep increases heart rate and insulin resistance
• Children are especially sensitive to light, with melatonin suppression occurring at very low light levels
• Sealed track technology, like that in the Ultimate Blackout Shade, eliminates the gap problem by guiding shade edges through wall-mounted channels
• Long-term light exposure during sleep is associated with higher rates of obesity, diabetes, and cardiovascular problems

Ready to experience what complete darkness feels like? Explore the Ultimate Blackout Shade or book a free consultation to discuss your specific needs.

Frequently Asked Questions

Do blackout curtains block 100% of light? No. While blackout curtain fabric blocks 85–99% of light passing through it, gaps at the edges, top, bottom, and center allow light to enter the room. Achieving complete darkness with curtains requires additional modifications that most homeowners find impractical.

Why do I still see light around my blackout curtains? Light enters through the gaps between the curtain and the wall. Because curtains hang from a rod rather than sealing against the window frame, spaces remain at the sides, top, and bottom. Air movement and fabric weight also create variable gaps throughout the day.

What is the difference between blackout blinds and blackout curtains? Blackout blinds mount closer to the window frame, reducing gap size compared to curtains. However, standard blinds still leave small gaps at edges. Shades with sealed track technology, like the Ultimate Blackout Shade, eliminate gaps entirely by guiding the shade through wall-mounted channels.

How much light is too much for sleep? Research suggests any light exposure during sleep can affect health markers. Studies show melatonin suppression at levels as low as 5 to 10 lux, which is dimmer than typical room lighting. For optimal sleep, aim for complete darkness.

Are blackout shades worth the investment? For anyone concerned about sleep quality, metabolic health, or circadian rhythm, the research supports complete darkness as beneficial. Shades with sealed track technology represent a higher upfront cost than curtains but eliminate the need for additional light-blocking solutions and their associated ongoing expenses.

Can I use blackout blinds with curtains? Yes. Many homeowners install the Ultimate Blackout Shade for light control and add decorative curtains in front for aesthetics. This layered approach provides both complete darkness and design flexibility.