What different light colors at night do to your body

Your body clock uses the light reaching your eyes to figure out whether it's day or night. Daylight has a lot of blue in it, so blue and cool white light late in the evening sends your clock much the same signal as daytime. Red and amber light send a much weaker signal, though they can still affect you if they're bright enough or on for long enough.

Why blue light at night affects melatonin

Melatonin starts to rise in the evening, a couple of hours before your usual bedtime, and stays up through the night. Light at night can lower it while the light is on, and it can move your body clock later so the next night's rise comes later too.

Not every color does this equally. In two studies published in 2001, researchers shone single colors of light into people's eyes at night and measured how much each one lowered melatonin. The strongest effect came from blue light, around 460 nm ¹². Green and yellow light, which look brightest to us, had less effect. Later work with longer exposures puts the most sensitive point a little higher, around 480 nm, a blue with a slight green tint ³.

In a 2003 study, people sat for six and a half hours at night under either blue (460 nm) or green (555 nm) light, matched for the number of photons. Blue light pushed their melatonin rhythm about three hours later. Green pushed it about an hour and a half later, and blue also kept melatonin down for about twice as long ⁴. Blue light at night also made people more alert than green in a similar comparison ⁵.

Ordinary white light contains blue. Cool white LEDs, daylight bulbs and most screens have more of it than warm white bulbs at the same brightness. If you're choosing bulbs, are LED lights bad for sleep goes through it.

Is red or amber light better at night?

Red and amber light have less effect on your body clock than blue or white light of the same brightness. The cells that tell your clock about light respond very little to red and amber. In one study, an hour of dim red light (40 lux) during the night didn't lower melatonin, while the same level of blue light did. Both colors did raise cortisol, a stress hormone that's normally highest in the morning, compared with sitting in the dark ⁶.

Red bulbs can still have an effect if they're very bright or left on for hours. Red light reaches the cones in your eye, which also send signals to the clock, and enough light of almost any color will have some effect. Amber is in between. Pure amber light has very little effect on the clock, but most amber bulbs and filters let some blue and green through.

Warm white (2700K or lower) is a reasonable middle ground for a home. It's bright enough for normal evening use, and it has noticeably less of the light the clock responds to than cool white.

Does brightness matter more than color at night?

The effect of light at night depends on how bright it is at your eyes, its color, how long it lasts and what time it is. Lowering any of these lowers the effect, so a few minutes of a dim night light does very little compared with an evening under bright ceiling lights.

Time of night matters because of how the clock responds. Light in the evening and the first part of the night pushes your clock later. Light in the last few hours before your usual wake time and after it pushes it earlier ⁷. So a lamp you switch on close to your bedtime has more effect than the same lamp earlier in the evening. If you want to use this to wake up earlier, the morning sunlight guide explains how.

Ordinary room light is bright enough to delay melatonin in most people ⁸, and some people respond to much dimmer light than others ⁹. A warm bulb turned up high can affect your clock more than a cool one turned down low. How much daylight you got earlier in the day changes this too, which the guide to daytime light and sleep explains.

The 2022 recommendations for evening light use a unit called melanopic lux, which counts light by how strongly it affects the clock, so color is already built in ¹⁰. The melanopsin guide explains the unit and the numbers, and the evening light guide has room-by-room changes.

Do blue-blocking glasses work?

Orange or amber glasses worn in the evening cut out most blue light before it reaches your eyes. In a study of simulated night shifts, people wearing them under bright light kept their melatonin at about the level they had in dim light ¹¹.

Evidence that they improve sleep is weaker. A 2021 systematic review found a small number of studies, mostly in people with sleep or mood problems, many with small samples and mixed results ¹². Some studies showed better sleep, others didn't. There isn't enough to say they help people who already sleep fine. Clear lenses sold as "blue light filtering" block far less blue than orange ones.

Glasses also do nothing about how bright the room is. Turning the overheads off and using a warm lamp lowers both the brightness and the blue, and it works for everyone in the room. Glasses make most sense when you can't control the light, like a bright office late at night, a shared space or a night shift. If you wear them, wear them in the evening only, because during the day they block light your clock uses to stay on time.