Blackout curtain for the bedroom: why total darkness changes your sleep

What the science says: light, melatonin, and sleep

The human body operates on a 24-hour circadian rhythm. When light diminishes, the pineal gland produces melatonin, the hormone that prepares the body for sleep. When light is present, even dimly, melatonin production is inhibited.

A study published in the Journal of Clinical Endocrinology & Metabolism (Gooley et al., 2011) demonstrated that exposure to 5 to 10 lux in the hours leading up to bedtime reduces melatonin production by 50% and delays its peak by 90 minutes. In practice, this means you take longer to fall asleep, your deep sleep phases are shorter, and you wake up feeling less rested.

For reference, here’s what common lux levels represent:

• 0.1 lux: full moon night. Optimal sleep condition.
• 1 to 5 lux: halo from a streetlamp through a thick curtain. Threshold at which melatonin is affected.
• 10 to 20 lux: light filtering through a sheer curtain. Enough to wake a baby.
• 50 to 100 lux: dimly lit room. Almost complete suppression of melatonin.
• 10,000 to 100,000 lux: daylight. Wake-up signal for the brain.

The effect is even more pronounced in summer when the sun rises at 5:30 AM and morning light reaches 10,000 lux by 6 AM. The WHO recommends less than 5 lux in the sleep environment. A 100% blackout curtain brings the bedroom down to less than 1 lux, even in broad daylight.

95% blackout vs 100%: why the difference matters

Many curtains sold as "blackout" actually block 85 to 95% of the light. On paper, 95% seems excellent. In practice, the remaining 5% makes all the difference for sleep.

In full summer sun (100,000 lux outside), 5% residual light still represents 5,000 lux in the bedroom, which is 1,000 times the melatonin suppression threshold. Even through a "near-blackout" curtain, your brain receives enough light to inhibit melatonin production and trigger a wake-up signal.

Concretely, 5% residual light means: a visible halo around the curtain, a strip of light at the bottom if the curtain is too short, and a yellowish tint through the fabric when the sun hits directly. This is not darkness. It is a dimness that leaves your brain in a state of partial alertness.

The problem stems from the lack of regulation. In France, the term "blackout curtain" is not controlled. Any thick curtain can be labeled "blackout" without testing or certification. To understand the differences between blackout levels, consult our detailed comparison on the difference between blackout, dim-out, and opaque curtains.

The 3 sources of light leakage in a bedroom

Even with a fabric that blocks 100% of the light, complete darkness is not guaranteed if the curtain is improperly sized. Light always finds a way.

• The sides: if the curtain does not extend 15 to 20 cm beyond the window on each side, light will seep in between the curtain and the wall. This is the most frequent and easiest leak to correct with a custom-made curtain of the right dimensions.
• The bottom: a curtain that stops 5 or 10 cm from the floor lets in a visible band of light from the bed. For a bedroom, the curtain should extend to the floor with a slight excess of 1 to 2 cm.
• The top: if the curtain rod is too close to the top of the window, light will pass over the curtain. The rod should be positioned at least 10 cm above the frame, ideally 15 to 20 cm. In homes without shutters, ceiling mounting may be the most effective solution.

Customization solves these three problems. A curtain made to the exact dimensions of your window, with the recommended overlaps, eliminates leaks that standard sizes cannot address. Consult our complete guide on how to measure your curtains before ordering.

Which blackout curtain for your situation

For a standard adult bedroom, the goal is complete darkness at night and control of morning light. A 100% blackout curtain in a calming shade does the job. A Travelodge/Sleep study (2013) of 2,000 households shows that people sleeping in blue bedrooms sleep an average of 7h52 per night, compared to 6h35 in brown bedrooms.

For a baby's room, the challenge is twofold: darkness for daytime naps and nighttime protection. Babies wake up from 10 to 20 lux, a threshold well below what a "near-blackout" curtain lets through. Custom-made is essential to eliminate any side leakage. Light shades (cream, mint green) work just as well as dark shades if the fabric is a true 100% blackout*.

For a night shift worker, sleeping in broad daylight is a constant challenge. The blackout curtain must create complete darkness at noon when the sun is at its zenith (100,000 lux outside). This is the ultimate test. Combining the curtain with sound insulation (22 dB* to block daytime street noise, construction, traffic) further improves rest quality.

For accommodation without shutters, the multi-layer blackout curtain is the only solution to create darkness. It completely replaces the function of a shutter, with the added bonus of thermal insulation (up to 7 °C gain*) and acoustic insulation (22 dB*). Many recent apartments, lofts, and modern houses are designed without shutters.

The color of the blackout curtain: a false criterion

A persistent belief is that a blackout curtain must be black or dark to block light. This is false. Blackout depends on the multi-layer structure of the fabric, not its surface color.

A Kurtens cream blackout curtain blocks exactly as much light (100%*) as an anthracite grey curtain, because it is the multi-layer construction of the fabric (620 g/m2) that prevents light from passing through, not the visible shade. All 7 Kurtens colors (grey, off-white, cream beige, mint green, green, lilac, blue) offer 100% blackout*.

You choose the color solely based on decoration and ambiance criteria. For a baby's room, a cream or mint green creates a soft atmosphere. For an adult bedroom, a blue or grey brings calm and elegance. Consult our article on what color curtain to choose for your room.

Blackout only or multi-layer blackout: what to choose

A "just" blackout curtain blocks light. But in a bedroom, other factors affect the quality of rest: noise and temperature.

Outside noises (traffic, neighbors, early morning birds) are the second leading cause of premature awakenings after light. A classic blackout curtain (150-300 g/m2), often thin despite its label, only attenuates 3 to 7 dB. A high-density multi-layer curtain (620 g/m2) attenuates up to 22 dB*, meaning four times less perceived noise.

Temperature also plays a role in sleep. The WHO recommends 16 to 18 °C for sleeping. A bedroom that is too hot in summer or too cold in winter disrupts sleep cycles. A thermal blackout curtain creates a barrier between the window and the sleeping area, with a measured gain of up to 7 °C*.

Kurtens custom-made blackout curtains combine all three functions in a single product. Available in 7 colors, with a maximum height of 270 cm and a custom configurator accurate to the centimeter. Free delivery in metropolitan France, Belgium, Switzerland, and Luxembourg.

*Data from tests conducted under optimal conditions.

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