Excessive net-surfing and television are leaving 12-year-olds suffering the symptoms of chronic sleep deprivation, say experts.

And this could, in the worst cases, have a detrimental effect on child development.

Many children now have television and computers even in their bedrooms, and are allowed to stay up late using the internet.

Researchers at Tel Aviv University looked at 140 Israeli schoolchildren aged eight, 10 and 12. Most were from "middle class", or "upper middle class" families.

Each child was given an actigraph, a tiny wristwatch-sized device which allowed continuous monitoring without interfering with sleep.

They found that the younger children were asleep on average an hour earlier than the older children.

And the older children, questioned about their levels of tiredness, reported feeling much drowsier during mornings.

The authors wrote: "This suggests the age related significant delay in sleep onset and the shortening of sleep leads to chronic partial sleep deprivation and increased day-time sleepiness even in this age group preceding adolescents, where such a tendency has already been established."

Physiological harm

They added that the sleep the 12-year-olds are getting may not "be in accordance with their physiological needs".

In adolescents, hormonal changes lead to later sleep times, but this does not explain the same effect in younger children.

Lead author Dr Avi Sadeh said there were several possible explanations.

He said: "There are increased school demands, the need of children to feel more like adults by having a more active night life and the incentives like late-evening or late-night TV shows and internet surfing."

The research also showed that younger parents tended to enforce earlier bedtimes.

The best way of predicting the quality of sleep, rather than the time spent asleep, were the parents' education and the general stresses within the family.

Children of parents with better education tended to sleep better.

The research was published in the journal, Developmental Psychology.