Many smartphone screens switch to warmer colours in the evening to help you sleep better – but research suggests the science behind this is all wrong

‘Night mode” is one of those features you may be aware of only because your phone keeps telling you about it. At some point while you are lying in bed at night sending texts, your screen may politely suggest you activate a function that shifts the colours of your screen from the colder to the warmer end of the spectrum. It is supposed to help you sleep better.

Findings in a study led by Dr Tim Brown and published in Current Biology suggest this is the very opposite of correct. The research, carried out on mice, appears to rubbish the notion that blue light disrupts sleep. All things being equal, warm yellow light is worse.

So, where did the idea of limiting blue light from screens originate? “There was definitely a valid scientific idea about why those things would work,” says Brown. It started about 20 years ago with the discovery of the role that melanopsin – a light-sensitive protein found in the eye – plays in regulating the body clock.

“The melanopsin system is fundamentally there to detect brightness,” says Brown. Because melanopsin was better at detecting short-wavelength photons, he says, it was thought it was biased in favour of blue light. In fact, it is the retina’s cone cells that determine colour. “The cone system also plays a role, and they’re doing the opposite of what most people thought,” says Brown.

According to the study, brightness levels are more important than colour when it comes to stimulating the body clock. However, when the light is equally dim, blue is more relaxing than yellow.

This makes basic sense: daylight is yellow, twilight is blue, and sunrise and sunset are pretty reliable ways to tell your body clock what time it is. Of course, at this point, we only know it works on mice – and mice don’t have phones. “We think there is good reason to believe it’s also true in humans,” says Dr Brown.

There is perhaps a more obvious truth to be drawn – if your phone is telling you to switch to night mode, it is time to put down your phone.

It is not the colour of the screen that is keeping you awake; it is all the stuff your phone offers as an alternative to sleep at 2am. There is only one real night-mode switch: the off button.