London Telegraph 
Sept 10, 2012
More and more people are taking their tablets to bed with them to surf the web, check Facebook or email before switching off the light.
But researchers are warning that the blueish light their screens emit can stop users getting a good night’s sleep.
That is because this type of light mimics daylight, convincing the brain that it is still daytime.
Blue light suppresses production of a brain chemical called melatonin, which helps us fall sleep. This is because our brains have evolved to be wakeful during daylight hours.
By contrast, light which is more orange or red in tone does not suppress melatonin production, perhaps because our brains recognise it as a cue that the day is ending.