LONDON (BBC Health News) -- A lack of sleep may be partly to be blame for childhood obesity, a leading UK expert claims.

Dr. Shahrad Taheri and his team at Bristol University say sleep deprivation leads to hormonal changes.

They say these changes tell the body to eat sugary or starchy food to provide an energy boost. Over a prolonged period this leads to weight gain.

Experts want the government to take sleep-deprivation as seriously as diet and exercise in the obesity battle.

The researchers at Bristol University say the evidence is stark. --------------------Energy imbalance Dr. Taheri told the BBC: "There is a really clear relationship between short sleep duration and obesity in children."

He said obesity occurs because of the "imbalance between the energy that we take in -- the foods that we eat -and the energy we burn, which is our physical activity".

Dr. Taheri added "there is a very good relationship between physical activity and good sleep".

The expert said this link was due to the fact that physical activity would make children tired, whereas youngsters were less likely to sleep well if they had been watching television or playing computer games.

He went on to say that children were "smart enough to learn about healthy foods and the importance of exercise", before adding that "we really should educate them about the importance of sleep".

One proposal is to start the school day later, so youngsters get a lie-in.

Another is to persuade parents to remove computers and mobile phones from their children's bedrooms, so they go to sleep on time.