Some youngsters may not be getting enough sleep, with almost half of teenage girls admitting they are not getting what they need to stay alert at school, research suggests.

While two thirds (63%) of 12- and 13-year-olds (Year Eight) say they are getting enough sleep for their studies, this drops as they get older.

Almost half (48%) of 14-15 year-old girls (Year 10) say they do not get enough sleep to stay alert and concentrate on their school work, according to a study by the Schools Health Education Unit. Two fifths (41%) of 14-15-year-old boys say the same.

The study, based on data gathered on more than 83,000 children in Years Six, Eight and 10 in 2010, also reveals that the proportion of youngsters getting eight hours or more of sleep a night drops with age.

Four fifths of 12-13-year-olds got this amount of sleep, compared to 61% of 14-15-year-old girls and 66% of boys of this age. And almost half (48%) of the youngsters questioned said they do not normally get enough sleep for their health.

The same study reveals that children are spending more time in front of the TV or computer than doing homework. Almost a quarter of boys (23%) in Years Eight and 10 spent more than two hours playing computer games the day before they were surveyed, while around 6.5% of girls said the same.

One in seven of the Year Eight and 10 pupils (14%) questioned said that they spent over two hours watching TV. But only 3% said they spent this amount of time on homework, with a third (36%) saying they spent no time at all on it, and 41% saying their homework took up to 30 minutes.

Cathy Ranson, editor-in-chief of parenting website Netmums.com, said: "The findings in this study make interesting reading for parents, particularly the fact that a very large proportion of young people admit to not getting enough sleep for them to stay alert and concentrate at school."

The report also found that a third of girls aged 10 to 11, and 29% of those aged 12 to 13 are afraid of going to school because of bullying at least sometimes. Girls are more fearful than boys, the report noted, and the older they get the less afraid they become.

Researchers also found that Year 10 boys are the most likely to be bullies themselves. And 6% of boys aged 14 to 15 said others may fear going to school because of bullies, while 10% admitted they had bullied someone last year.