Firelight is orange: what does humanity's evening light say about your lamps today?

How long have humans had light in the evening?

Electric light at home is just over a hundred years old. Fire is far older. According to a review of the archaeological finds in Europe, published in PNAS in 2011, habitual use of fire appears around 300,000 to 400,000 years ago. Our eyes and internal clock developed with a single evening light: fire.

Before that there was the moon and the stars. After it came oil lamps and candles, which are fire too. For almost all of our history, light after sunset has been dim, warm and orange.

Why is fire orange?

Its colour depends on the temperature of what is glowing. The colour of light is measured in kelvin. A candle flame sits around 1800 K, a classic incandescent bulb around 2700 K, and daylight from 6000 K upwards. The lower the number, the more of the light falls in yellow, orange and red, and the less in blue.

That matters because the eye's internal clock reads exactly the blue. The cells that set the clock contain the protein melanopsin and are most sensitive around 480 nm. We cover them in Your eye's third cell type. Fire gives off almost no light there. To the clock, a campfire is close to darkness.

What happened when people only had sun and fire?

In 2013, researchers at the University of Colorado published a study in Current Biology. Eight people first lived a week with ordinary electric light at home and at work. Then they camped for a week in the Rocky Mountains in July, with only sunlight and campfire, no flashlights and no phones.

During the camping week they got more than four times as much daytime light, 4487 lux on average against 979 lux at home. Their melatonin, the hormone that marks the body moving into night, came about two hours earlier and sat on average close to sunset. Melatonin ended just after sunrise, instead of after they woke up.

The study is small, was done in summer, and did not measure whether the participants felt better. But it shows how closely the internal clock follows light. With sun by day and fire by night, it set itself to the sun within a week.

How is today's evening light different from fire?

Most white LED lamps and screens create their light with a blue diode. That gives them far more blue than a flame, even when the light looks warm. And we usually sit closer to the light source, for more hours, than anyone ever sat by a fire.

The mechanism, how evening screen light holds melatonin back, is covered in Blue light and melatonin.

Why is the LOWLIGHT evening lens orange?

Because it does roughly what fire always did. It lets yellow, orange and red through and filters out the blue. The orange lens filters 98% of light between 400 and 500 nm according to the supplier's spectral measurement. Colours in the room turn warmer, as in firelight, but you still see the screen and the faces around you.

For the last two hours before sleep there is the red lens, which filters 99% between 400–550 nm. That is the lenses' filtration, not a promise of better sleep. Fire was never a sleeping pill either. It was simply the right kind of light for the time of day.