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The colour cyan - between green and blue - is a hidden factor in encouraging or preventing sleep, according to biologists.

University of Manchester researchers say higher levels of cyan keep people awake, while reducing cyan is associated with helping sleep.

The impact was felt even if colour changes were not visible to the eye.

The researchers want to produce devices for computer screens and phones that could increase or decrease cyan levels.

Sleep researchers have already established links between colours and sleep - with blue light having been identified as more likely to delay sleep.

There have been "night mode" settings for phones and laptops which have reduced blue light in an attempt to lessen the damage to sleep.

But the research by biologists at the University of Manchester and in Basel in Switzerland, published in the journal Sleep, has shown the particular impact of the colour cyan.

When people were exposed to more or less cyan, researchers were able to measure different levels of the sleep hormone melatonin in people's saliva.

Prof Rob Lucas said that it was not necessary for someone to be able to see the difference in colours, as the body reacted to the change even if it was not visible to the naked eye.

He said this could also affect other colours which were made using cyan.

For instance, there are shades of green that can include cyan - which also can be achieved using other colour combinations.

The researchers suggest that versions of the colour using cyan could be used on computer screens if the aim was to keep people awake - such as people working and required to stay alert at night.

Or there could be another version, the same colour but without cyan, which could be used if the aim was reduce disruption to sleep.

The research used this with a movie - with the colours being adapted to include or exclude cyan - and found changes in viewers' sleepiness and levels of melatonin in saliva.

The research team, headed by Prof Lucas and Dr Annette Allen, says there could be applications for this discovery on computer screens, televisions and smartphones.

"This outcome is exciting because it that tells us that regulating exposure to cyan light alone, without changing colour, can influence how sleepy we feel," said Prof Lucas.

He said it might help families with teenagers who were using mobile phones at night-time.