The use of tablet computers just before going to bed can cause a poor night's sleep, a study has revealed.
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, the Telegraph reported.
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, the report said.
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, it added.
Neurologists have known for years that staring at screens late in the evening can disrupt sleep - be it televisions, computers or mobile phones.
However, because mobiles and tablets are by nature portable - not to say addictive - more people are taking them into the bedroom.
Users also tend to hold them much closer to their eyes than a computer or television screen.
Researchers at the Lighting Research Centre, at the Rensselaer Polytechnic Institute in New York, are now warning that looking at tablet displays for more than two hours "leads to a suppression of our natural melatonin levels as the devices emit optical radiation at short wavelengths" - in other words, they emit bluer light.
"Although turning off devices at night is the ultimate solution, it is recommended that if these devices are used at night displays are dimmed as much as possible and that the time spent on them before bed should be limited," they said.
They findings are published in the journal Applied Ergonomics.