Being on your phone before bed may not prevent you nodding off, research suggests.
The blue light emitted by electrical devices was long thought to suppress the “sleep hormone” melatonin by “tricking” the mind into thinking its daytime.
A new study by the University of Manchester suggests it all comes down to how warm and bright the light is.
Scientists exposed mice to a range of coloured lights of the same brightness.
Results, published in the journal Current Biology, show bright light of either a blue or yellow hue was equally as “stimulating” for the animals.
When the light was dimmed, however, the blue hue was more restful than the yellow.
Our body clock determines night and day via a light-sensitive protein, called melanopsin, in the eye.
Melanopsin is better at detecting rays of light with a short wavelength, like the aquamarine spectrum.
It was therefore thought to be more sensitive to blue light, with the glowing screens of our phones and tablets throwing our body clocks “off whack”.
The scientists now believe the cells in our eyes that interpret colour, retinal cone cells, supply “blue light signals” that lessen the impact on our body clock.
“We show the common view that blue light has the strongest effect on the clock is misguided,” lead researcher Dr Tim Brown said.
The results appear to mimic the natural rising and setting of the sun.
“During the daytime, the light that reaches us is relatively white or yellow and has a strong effect on the body clock”, Dr Brown told the BBC.
“Around twilight, once the sun sets, the bluer the light becomes”.
Many turn down the “blueness” of their screens before bed in the hope it will help them nod off. The results suggest, however, the opposite may be true.
“It's counteracting any benefit you might get from also reducing the brightness of the screen,” Dr Brown said.
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Research is looking into how to change the brightness of screens to promote sleep.
“There is lots of interest in altering the impact of light on the clock by adjusting the brightness signals detected by melanopsin,” Dr Brown said.
“Current approaches usually do this by changing the ratio of short and long wavelength light. This provides a small difference in brightness at the expense of perceptible changes in colour.
“We argue this is not the best approach, since the changes in colour may oppose any benefits obtained from reducing the brightness signals detected by melanopsin.
“Our findings suggest using dim, cooler lights in the evening and bright, warmer lights in the day may be more beneficial.
“Research has already provided evidence that aligning our body clocks with our social and work schedules can be good for our health. Using colour appropriately could be a way to help us better achieve that.”
Another expert stresses, however, further studies are required.
“This is fascinating work but we really don't know yet the same happens in humans,” Dr Manuel Spitschan, from Oxford University, told the BBC.