Using iPads at bedtime can lead to poor night's sleep

The study found that a large number of people are taking their tablets to bed with them to surf the web, check Facebook or email before switching off the light.

Researchers, however, have warned that the blueish light emitted by the screens can stop users getting a good night's sleep, 'The Telegraph' reported.

It 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.

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 these gadgets much closer to their eyes than a computer or television screen.

Researchers from Lighting Research Centre, Rensselaer Polytechnic Institute in New York, warn 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".

"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 drew their conclusions after measuring melatonin levels in 13 volunteers, after they had spent time viewing iPads at full brightness at a distance of 10 inches, for two hours.

The findings are published in the journal 'Applied Ergonomics'.