What is known, and how precisely
The retina does more than produce vision. It contains melanopsin-carrying ganglion cells, most sensitive around 490 nanometres in the blue-cyan range, which tell the internal clock what time it is outside. That circuit is what suspends the night-time release of melatonin when light arrives.
The landmark work of the early 2000s established a dose-response relationship, and the surprise was how low the relevant levels sat: a substantial share of the maximum effect is reached at illuminance most people would call dim, well below normal living room lighting. More recent work added a qualification that matters: sensitivity varies enormously between individuals, by a factor of more than fifty from the least to the most sensitive, and it depends on the light received earlier in the day. A day spent outdoors makes the evening more forgiving. A day in a dim office makes it more fragile.
Two practical consequences follow. First, there is no universal threshold to quote, and anyone offering a single number should prompt caution. Second, since a few lux are enough for sensitive people, aiming as low as reasonably possible costs little and risks nothing. Measurement belongs at eye level, not on the desk. Closed eyelids attenuate light by one to two orders of magnitude depending on wavelength, but they do not cancel it.
Measuring at the sleeping position
A light meter costing 30 to 80 euros reads down to 0.1 lux and is sufficient. Place it on the pillow, sensor facing the ceiling, then turn it towards the suspected main source, with shutters closed and the rest of the home dark as it would be at three in the morning. Phone apps give an order of magnitude above 20 lux and are worthless below 5.
| Situation | Order of magnitude |
|---|---|
| Moonless night, far from any town | 0.001 lux |
| Full moon, clear sky | 0.1 to 0.3 lux |
| Well blacked-out city bedroom | 0.1 to 1 lux |
| Blue LED standby light two metres away | 0.5 to 3 lux |
| Bright alarm clock display at 50 cm | 1 to 5 lux |
| Street lamp through a light curtain | 3 to 20 lux |
| Lit hallway, door ajar | 20 to 60 lux |
| Phone screen held at 30 cm | 40 to 100 lux |
Three reading points: below 1 lux at the pillow there is little left to gain. Between 1 and 5 lux the room is already in decent shape. Above 10 lux there is an identifiable source, and finding it takes five minutes of dark adaptation and a look around.
Stray sources, in descending order of yield
- Standby lights. Router, television, chargers, air purifier, thermostat. A strip of matt black tape settles each one in ten seconds. It is the cheapest correction anywhere in our framework.
- The alarm clock. Amber or red display, minimum brightness, turned to face the wall. Ceiling-projection clocks have no place in a bedroom.
- The phone. Out of the room, or face down on the table. The problem is not only optical.
- Night-time transit lighting. One trip to the bathroom under a ceiling light undoes the benefit of the night. A motion-activated amber night light at floor level, around 1 to 3 lux, solves it without waking the clock.
- The outside. Street lamps, signage, courtyard lighting. This is the only item that requires hardware.
Real blackout versus a curtain sold as blackout
A curtain marketed as blackout stops most of the light passing through the fabric and lets the rest in around the sides, over the top and under the hem. The leak is geometric, not textile, which is why heavier cloth changes almost nothing until the edges are dealt with.
What actually works, by cost:
- a sleep mask, 5 to 20 euros, immediate, portable, and by a wide margin the best effect for the money. We say so plainly, even though it is not a piece of design work. It does not suit everyone and does nothing for the early light that wakes the other occupants of the home
- a track extended 15 to 20 cm either side of the window, with a wall return and the curtain reaching the floor, roughly 100 to 250 euros
- a roller blind with side channels, which blocks the edges by construction, 150 to 400 euros per window, and the cleanest interior solution
- an external roller shutter with seals, 400 to 900 euros, which also helps against noise and summer heat, and that changes the arithmetic.
The morning counterpart
Darkness at night only pays in full if the day opens with strong light. The scale gap is the central fact: a well-lit interior delivers 200 to 500 lux, an overcast winter sky 2,000 to 20,000 lux, a clear summer day up to 100,000 lux. No domestic fitting competes in that range.
Twenty to thirty minutes outdoors within two hours of waking, without sunglasses, is enough to deliver the signal. In winter at northern latitudes, a 10,000 lux light therapy lamp used for twenty to thirty minutes in the morning is a documented option, best established for seasonal depression. It does not suit everyone: bipolar disorder, retinal conditions and photosensitising medication call for medical advice first, and that advice is not ours to give.
What belongs to the address and what belongs to the room
Part of the problem is settled before anyone moves in. Light pollution is mapped, and the gap between a facade on a main road and a bedroom over an inner courtyard cannot be recovered with a curtain. French rules on light nuisance have required night-time switch-off of building facades and many shop windows since 2018, and more than half of French municipalities now turn off some or all of their street lighting for part of the night. That is worth asking the town hall, and worth checking on site, one evening, at one in the morning.
The rest belongs to the room, and that is the encouraging part: standby lights, the clock, the door and the window edges account for most of the available gain, at a cost that rarely exceeds the price of a roll of tape. We measure, we cost, we rank. Lif8 is not a healthcare provider: settled insomnia is treated with a doctor, not with a light meter.
