How we tested Sundown: blue light and screen flicker

Why we tested Sundown

A screen is a light source at eye level, and most of us sit in front of one for hours after dark. When researchers measured a tablet, an e-reader and a phone, all three gave off light with a strong peak in the blue (Gringras et al., 2015), the part of the spectrum the eye’s clock sensor responds to most. The science behind that sensor is on the page about the eye’s clock sensor. Sundown exists to change what your Mac sends to it, so the obvious question was whether it does, and the honest way to answer it was to measure the light rather than describe the settings.

The second question was flicker. Many displays dim by switching the backlight on and off very fast, a method called pulse-width modulation, or PWM. Sundown has a flicker-free mode that is meant to take that switching out of the picture, and a claim like that is easy to make and easy to check, so we checked it.

The blue light test

Dr. Alexander Wunsch is a German physician who works on light and health, including the spectral quality of artificial light and flicker. He measured the light coming off a Mac screen with Sundown switched off and again with Sundown at deep warmth, and compared the two across the blue band, from 380 to 500 nm. Instead of picking out one wavelength, the measurement adds up all of the light across that band, so it answers the question that matters: how much blue light is the screen still sending to your eyes?

With Sundown off, the screen gave out the blue you would expect from an ordinary display. At deep warmth, the blue was gone. That is the result in plain words: at deep warmth, Sundown eliminates blue light.

What deep warmth means on your Mac

Sundown sets warmth on a colour scale, the same scale photographers and lighting designers use for the colour of light. Ordinary daylight white on a screen sits at the cool end. Night Shift stops at about the point where the screen looks a warm white. Sundown goes all the way down to its warmest, a deep red-orange, and the further down that scale you go, the less of the screen’s light falls in the blue band. Deep warmth is the warm end of that range, which is where the test found the blue gone.

Sundown also dims the screen, and that matters because the body clock responds to the total amount of light as well as its colour. A group of circadian researchers recommends keeping the light reaching your eyes to no more than 10 lux melanopic EDI, a unit weighted for the clock sensor, from at least three hours before bed (Brown et al., 2022). Taking the blue out and turning the light down together is how a screen gets anywhere near that, and it is why Sundown does both. The page on brightness, colour and melanopic lux goes into the measure itself.

The flicker test

Pulse-width modulation dims a backlight by switching it on and off many times a second and varying how long it stays on. The switching is far too fast to see as flicker, but it is still there, and the eye is not blind to it. In one study, the retina produced a measurable electrical response in step with flicker faster than people can see, from a computer monitor and from fluorescent lighting (Berman et al., 1991). In offices, when fluorescent tubes were switched to electronic ballasts that cut their flicker from a modulation of 43 to 49% to below 7%, reported headaches and eyestrain more than halved (Wilkins et al., 1989). That was lighting, not a laptop, but it is the reason flicker is worth taking seriously.

Sundown’s flicker-free mode deals with it by not asking the backlight to dim at all. It holds the built-in display’s backlight at full power and does the dimming in software, through the display’s colour tables, so the screen gets darker while the backlight stays steady. I tested the screen with a USB oscilloscope, with flicker-free mode on. No flicker. Sundown removes screen flicker.

If you want the background, the post on flicker-free mode explains the setting, and the page on PWM and invisible flicker covers the research.

What it means when you use Sundown

In practice it comes down to two settings. In the evening, take the warmth deep, because that is where the blue is gone, and let Sundown dim the screen as well, because less light overall means less signal to your body clock. Keep flicker-free mode on, and the backlight stays steady however dark you take the screen.

None of that asks you to stop using your Mac at night, which for most people was never going to happen anyway. It changes the light the Mac gives you while you use it.