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Why blue keeps you awake

Mobile phone and app designers have noticed something that you probably spotted before you started school. In the day, the sky is blue. In the evening it turns redder.

This is due to Rayleigh scattering: air particles divert blue light more than red light. If you’re into home experiments, you can test this. Stir some water and milk together in a glass to get a nice cloudy mixture. Then turn the lights off and shine a torch through the milk. Head-on, you’ll see orangey red, like the sun at sunset. From the side of the glass, the scattered light will seem blue.

But this is a psychology blog rather than a physics blog. Why should the geeks in Silicon Valley care? After all, they don’t get out much.

The reason is that light colour is one of the cues our bodies use to work out what time it is. In the day, we are perky and active. In the evening, it winds us down for a good night’s kip.

Our mobile screens are blue heavy. If you came here from Facebook, you’ll have noticed the colour scheme.

Researchers showed volunteers images on a screen between 6pm and 11pm. They subtly varied the colours in the display. They asked the participants how sleepy they felt, and took saliva swabs.

When the scientists increased the amount of blue light in the display, the participants claimed to be more alert. The swabs told the same story: the hormones which send us to sleep were markedly higher when there was less blue light on the display.

This isn’t to say that app and mobile phone designers are quite as crafty as this makes them seem. They might not have made the connection between the day’s cycle and the light-waves their devices emit.

But we can assume that, being scientifically inclined, they’ve done some A/B testing of their livery. They might not have realised why the blue theme worked better than the red one, but still gone with the colour scheme that kept us actively clicking long after we should have turned the lights out.

Chris Paley is the author of Unthink, which has sold over 30,000 copies in English and been translated into five other languages. One magazine said it will make you ‘happier than a rabbit on a carrot farm’. If you're not too sleepy, and want to be that happy, buy it here.


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