Using tablet computers like Apple's iPad and Samsung's Galaxy¹ Note just before bed can lead to a poor night's sleep, according to research.

 

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 mimics² daylight, convincing the brain that it is still daytime.

 

Blue light suppresses production of a brain chemical called melatonin（褪黑色素）, 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 recognize 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 they television screens, computer screens or mobile phone screens.

 

However, because mobiles and tablets are by nature portable - not to say addictive³ - 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 Lighting⁴ Research Centre, at the Rensselaer Polytechnic⁵ Institute in New York, are 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 wavelengths⁶" - in other words, they emit bluer light.

 

They say: "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 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.

 

Melatonin levels were significantly lower after they had done this, than they were after the volunteers had viewed their iPads for the same time, but while wearing orange glass goggles⁷, which cut out the blue light.

 

They wrote in the journal Applied⁸ Ergonomics that tablet makers⁹ could "tune¹⁰ the spectral¹¹（光谱的） power distribution of self-luminous devices" so that they disrupted the sleep patterns of users less.

 

It is not just a good night's sleep that could be jeopardized¹²（危害） by too much late night screen time.

 

Researchers know that persistent¹³ disruption to sleep patterns can lead to an increased risk of obesity¹⁴, and even breast cancer.

 

However, these studies tend to be comparisons of those with chronic¹⁵ sleep disruption, such as long term shift workers, with those who have normal sleep patterns.

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