Many teenagers do not believe they are getting enough sleep to remain alert at school and stay healthy, research suggests.
It reveals girls are more concerned about their sleeping habits than boys, and that youngsters are more likely to say they are not getting enough as they get older.
More than one in four 14 and 15-year-old girls (28%), and just over a fifth of boys of the same age (22%) do not think they sleep enough to concentrate on their studies, according to the Schools Health Education Unit.
Their findings, drawn from surveys of thousands of schoolchildren aged from 10 to 15, show that fewer 12 and 13-year-olds (Year 8) are concerned about lack of sleep affecting their classwork.
A fifth (20%) of Year 8 girls, and 16% of boys said that the amount of sleep they normally get is not enough for them to stay alert and concentrate on lessons.
The research shows the proportions of youngsters who are concerned about the impact lack of sleep has on their health, with 17% of 12 and 13-year-old boys and the same number of girls saying they don't think get enough to stay healthy.
This rose to 22% among 14 and 15-year-old boys (Year 10) and 27% of girls of the same age.
Overall, 80% of Year 8 boys and 78% of Year 8 girls said that they get eight hours or more sleep a night, this fell 65% for Year 10 boys and girls.
The study also reveals that many youngsters are spending much more time playing the computer and watching TV than doing homework.
Asked how long they had spent doing homework the night before, 4% of 14 and 15-year-old boys said that they had spent more than three hours on it.
But almost a fifth (19%) of boys of this age said that they had spent more than three hours playing computer games, and 14% had spent more than three hours watching TV.
Among girls in the same age group, 10% had spent over three hours on homework, 5% had spent this long playing computer games, and 15% had spent over three hours watching TV.
Maggie Fisher, health visitor for parenting website Netmums said: "Children, and teenagers in particular, are leading increasingly busy lives and we know this is leading to a sleep deficit. Research shows for each hour of sleep lost, IQ drops by a point so the cumulative effect of lots of late nights can have a serious impact on teenager's studies at school.
"Children nowadays spend far more time surfing the web or watching TV, so sleep experts advise all electronic devices to be switch of at least 30 minutes before bedtime or it can affect sleep quality and brainwave patterns.
"However what we are seeing is many children sleeping with smartphones tucked under their pillows which prevents restful sleep."
She added that teenagers have different sleep patterns to other people, making them more likely to go to bed later and get up later.
Mike Griffiths, vice president of the Association of School and College Leaders (ASCL) said: "As this survey suggests, there's a general concern at the amount of time youngsters are spending interacting with screens, whether TV or computers, as opposed to interacting with people."
Mr Griffiths suggested that lack of sleep could be linked to the amount of time spent playing computer games or watching TV.
He said he was not sure if taking part in these activities makes children more or less tired than going outside and playing a game like football.
But he added: "Some of these videos and interactive games are not very restful. With physical activity perhaps you get a good night's sleep."
Mr Griffiths said that in some cases, it may be that parents are telling a child to go to bed, saying goodnight and then a little later the child is back up playing computer games in their bedroom.Suggest a correction