How to Make a Bedroom Dark Enough for Sleep

Darker than feels necessary. Melatonin suppression begins at light levels far below what reads as a dark room, and exposure to ordinary room light during sleep suppresses it by more than half in most people. Deal with the sources in order of cost: device LEDs, then curtain gaps, then the window itself — or skip all three with a mask.

The threshold is lower than it feels

Two findings set the scale, and both are more severe than intuition.

Room light before bed delays the whole night. Compared with dim light, ordinary room light in the hour before sleep pushed melatonin onset later in 99 percent of people tested and shortened the duration of melatonin release by around 90 minutes.

Room light during sleep is worse. Exposure to room-level light through the usual sleeping hours suppressed melatonin by more than half in 85 percent of trials.

The suppression threshold itself starts near 1.5 melanopic lux — a level most people would describe as darkness — with half of the maximum effect around 21 lux. For scale, 15 lux is enough to suppress melatonin in adolescents, and 15 lux is roughly a phone screen face-up on a bedside table.

The practical reading: a bedroom that feels dark is often not dark, and your sense of "dark enough" is calibrated by pupils that have already adjusted.

The sources, in the order they cost money

Free — tonight.

Every LED in the room. Router, TV standby, charger, air purifier, humidifier. Black electrical tape over each one takes five minutes and removes more lux than any curtain.
Phone face-down or out of the room. A screen lighting up mid-night hits the eye directly.
Close the door. Hallway light through a gap sits right at the threshold that matters.

Cheap.

The gap at the sides of the curtains. This is where nearly all street light enters, not through the fabric. A blackout curtain hung inside the recess leaks around the edges; the same curtain hung wide, outside the recess, does not.
A sleep mask. The only fix that works on the first night, in any room, including hotels and other people's houses. It also removes the light you cannot control — a partner's lamp, a streetlight installed after you moved in. What a mask does and does not do is in do sleep masks work.

Expensive.

Blackout blinds or a fitted frame. Effective, permanent, and the only real answer if you sleep during the day.

Our own devices are on this list

We sell a device that emits light in a bedroom, so we will say it plainly.

The Flame Mist Humidifier Diffuser is built around a flame-effect glow in seven tones — that is the point of it.

It is lovely in the hour before bed and belongs switched off once you are actually asleep. A flame effect glowing all night in a bedroom is a light source sitting exactly in the range the research is about. Use the timer, or run it in the living room and let the bedroom be dark.

That is the honest version. The flattering version would have been to leave this paragraph out.

What order to fix things in

Darkness is the second lever, not the first. Temperature moves sleep efficiency more, and it is usually cheaper to change — the measured ranges are in bedroom temperature and humidity. Get the room to 20–24 °C, then get it dark, then deal with humidity, and only then think about scent, which marks the hour rather than causing sleep. The full order is in what actually helps you fall asleep.

Questions people ask next

How dark should a bedroom be for sleep?

Darker than it needs to feel. Melatonin suppression begins around 1.5 melanopic lux — a level most people would call dark — and reaches half its maximum effect near 21 lux. Ordinary room light during sleep suppresses melatonin by more than half in 85 percent of trials.

Does a small LED really affect sleep?

A single standby LED is small, but the threshold is small too, and the effect is on melatonin rather than on whether you feel sleepy. Taping over the LEDs in a bedroom is free, takes five minutes, and removes more light than most curtain upgrades.

Do blackout curtains work?

The fabric works; the installation usually does not. Most light enters at the sides, so a blackout curtain hung inside the window recess leaks around its edges. Hang it wide and outside the recess, or accept a mask as the more reliable fix.

Is a sleep mask as good as a dark room?

For the eye, better — it blocks light the room cannot, including a partner's lamp and a streetlight you cannot move. It works on the first night and travels. It does nothing for the rest of the room, which is why it is a fix rather than a renovation.

Can I leave a diffuser light on all night?

We would not. A glow that runs through the night is a light source in the range this research is about. Use the timer, keep it to the hour before bed, or run the device in another room and leave the bedroom dark.

Is red light better at night?

Light with little power in the melanopic range suppresses melatonin less, which is why warm, dim light is a better bedside choice than a cool bright one. Less light is still better than warmer light.

Where these numbers come from

Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans (PubMed 21193540) — later melatonin onset in 99 % of individuals, duration shortened by ~90 minutes; suppression of >50 % in 85 % of trials during usual sleeping hours
Nocturnal melatonin suppression by adolescents and adults for different levels, spectra, and durations of light exposure (PMC6640648) — suppression threshold near 1.5 melanopic lux, half-maximum near 21, saturation near 305
Effect of exposure duration and light spectra on nighttime melatonin suppression in adolescents and adults (PMC6561500) — 15 lux sufficient to suppress melatonin in adolescents
Light Modulation of Human Clocks, Wake, and Sleep (PubMed 32342043) — spectral dependence: low melanopic content is less suppressive
Flame Mist light: seven glow colors, from the supplier's product listing

How we research and check these numbers