Why does your phone turn orange at night, and is Night Shift enough?

Why does the screen turn orange in the evening?

On the iPhone the feature is called Night Shift, and it arrived with iOS 9.3 in March 2016. Apple's own description is that it adjusts the colours of the display to the warmer end of the spectrum, and that by default it turns on at sunset and off at sunrise, using the clock and location in the phone. Android has an equivalent called Night Light.

Apple spelled out the reason when the feature was announced: many studies have shown that exposure to bright blue light in the evening can affect your circadian rhythms and make it harder to fall asleep. That is unusually plain language from a phone maker, and it is the same mechanism the whole LOWLIGHT line is built on.

What happens in the eye when the screen is blue?

The retina has a third kind of light-sensitive cell that forms no images but measures light and sends the signal to the brain's clock. It is most sensitive to blue. In a study from 2001, light between 446 and 477 nm was the most effective at switching off melatonin, the hormone that marks the start of the night. How the cell works is covered in Your eye's third cell type.

Screens hit that band almost exactly. When researchers at King's College London and the University of Surrey measured the spectrum of an iPad Air, an iPhone 5s and a Kindle Paperwhite in 2015, all three had their blue peak between 445 and 455 nm when showing text. A white screen in the evening is not just light in general. It is light in precisely the colour the clock listens to. What that does to a whole evening was shown by sleep researchers in Boston in PNAS in 2015: when participants read on a light-emitting tablet instead of a printed book in the hours before bed, they took longer to fall asleep, had less melatonin, a later body clock and were less alert the next morning.

Does Night Shift help? What the measurements show

The Lighting Research Center at Rensselaer tested the question directly. Twelve young adults used iPads between 11 pm and 1 am on four evenings, with Night Shift in two settings, one warmer and one less warm, plus a control evening in dim light and an evening of bright light for comparison. All three lighting conditions suppressed melatonin significantly over the two hours, and there was no significant difference between the two Night Shift settings. The researchers' conclusion in Lighting Research & Technology was that changing the spectral composition of a screen without changing its brightness may be insufficient to prevent an effect on melatonin.

A larger study from Brigham Young University measured sleep rather than the hormone. 167 adults aged 18 to 24 were randomised to three groups for seven nights: phone during the last hour before bed with Night Shift on, the same with Night Shift off, or no phone at all. Sleep was measured with wrist-worn accelerometers. There were no significant differences in sleep onset, sleep duration or sleep efficiency between the groups. In a post-hoc analysis of those who slept more than the median of 6.82 hours, the no-phone group had 88.78 percent sleep efficiency against 86.57 percent for the Night Shift group. The researchers pointed out that it is not only the light but also what you do on the screen that keeps you awake.

That does not make colour unimportant. It makes colour one of three things that count: what colour the light is, how much light there is, and what you are doing in it. Night Shift touches only the first, and only on a single screen.

What does an orange lens do that the screen cannot?

The same British research group also measured what happens when orange-tinted glasses are placed between the eye and the iPad. Melanopic light, the light weighted by how the clock cell perceives it, fell from 302 to 65 melanopic lux, roughly 79 percent. Visible light fell far less, from 319 to 202 lux, roughly 37 percent. The lens removed the blue and let the rest through.

The difference from Night Shift is where the filter sits. Night Shift changes one screen. A lens sits in front of the eye and filters everything you look at: the TV, the laptop, the tablet, the kitchen lights and the phone, whatever the app and whatever the brightness. That is why Viktor says the phone already turns orange at night, and still keeps the glasses on.

The lens figures are the supplier's spectral measurement per lens tint. The orange lens is meant for the evening on screens, the red for the last two hours before sleep. Which one suits which hour is covered in Yellow, orange or red lens.

How do you use Night Shift and a lens together?

Leave Night Shift on, it costs nothing. Turn the brightness down in the evening, since the measurements above suggest the amount of light counts as much as its colour. And for the last hours, when the TV and the laptop are lit as well, put the lens in front of the eye instead of trying to set every screen in the room. What a whole evening can look like is in The Evening Routine.

What none of these studies shows is that a lens by itself gives better sleep. The Brigham Young study is a reminder that what you do on the screen matters too. The lens changes which light reaches the eye. The rest of the evening is yours.