By Angela Harrison
BBC News education and family reporter
"I go on the XBox, then my PSP, then I watch TV" - Liverpool children on how much sleep they get
Video games, mobile phones and TV are keeping children up at night, answers to a BBC questionnaire suggest.
Newsround sent a questionnaire to 1,000 children aged between nine and 11 at schools across the UK.
Most said they went to bed at 2130, but a quarter said bedtime was 2200 or later and half said they were not getting enough sleep and wanted more.
Health experts have linked a lack of sleep to problems with concentration, behaviour and school work.
About half the children asked said they were staying up to play on computer games or their mobile phones or to watch television.
More than half of the children taking part said they had a television in their bedroom.
Lewis, who is 10 and from Liverpool, told Newsround he shares his bedroom and there are a lot of distractions at bedtime.
Bedtime is usually 2200 for Lewis
"I play on my games. It takes me until 10 o'clock to go to sleep. I am tired in the morning," he said.
For children aged 10, experts recommend at least 10 hours of sleep a night.
Of the children who filled in the questionnaire, 314 out of 1,083 said they went to bed at 2130, 272 said 2100.
A total of 277 said they stayed up until 2200 or later.
Scientists have linked a lack of sleep in children to problems with concentration and schoolwork.
Energy levels can be lower and sleep-deprived children can be irritable or behave badly.
A recent study by academics in Finland suggested a good night's sleep could reduce hyperactivity and bad behaviour among children.
They said adequate sleep could improve behaviour in healthy children and reduce symptoms of attention deficit hyperactivity disorder (ADHD).
There's a huge amount of brain development that's going on and we know that even moderate sleep loss impacts on their ability to concentrate and behave the following day
Paul Gringrass, Guy's and St Thomas' Hospital
They said it was estimated that a third of US children do not get enough sleep.
Paul Gringrass, a paediatrician who runs the children's sleep clinic at Guy's and St. Thomas' Hospital in London, said: "Children aren't just little adults. There's a huge amount of brain development that's going on and we know that even moderate sleep loss impacts on their ability to concentrate and behave the following day".
He said a lack of sleep affected children's growth and appetite too.
"Conditions like attention deficit hyperactivity disorder can manifest in children who simply haven't had enough sleep. There are certain hormones we produce more of when we've had a bad, disruptive night, which make us hungrier. And we have this obesity epidemic. It's a vicious cycle".
Teenage night owls
It is not just young children who are missing out on their sleep.
According to researchers, teenagers are suffering from what they call "night owl syndrome" because they do not get enough sunlight.
A study by the Rensselaer Polytechnic Institute's Lighting Research Centre in the USA concluded that teenagers were missing out on exposure to light, especially in the morning, and that this was upsetting their body-clocks.
The research was led by Mariana Figueiro, who said: "As teenagers spend more time indoors, they miss out on essential morning light needed to stimulate the body's 24-hour biological system, which regulates the sleep/wake cycle.
"These morning-light-deprived teenagers are going to bed later, getting less sleep and possibly under-performing on standardised tests."
The study was published in the journal Neuroendocrinology Letters.
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