And he says there is evidence that young children are getting too little sleep, with a detrimental effect on their behaviour.

BBC: NEWS | UK | Magazine | Why should we have eight hours' sleep?

The most common sleep problems in children are difficulty falling asleep and staying asleep and overall insufficient shut-eye.

WSJ: More Children With Sleep Problems Seek Overnight Tests

There are unique challenges in doing sleep studies on children.

WSJ: More Children With Sleep Problems Seek Overnight Tests

On any given night in America more than 640, 000 men, women and children are forced to seek shelter, live in their cars, or sleep on the streets.

ECONOMIST: A national plan to end a national disgrace

His prospects remain bleak, but at least he has a bed to sleep in each night -- unlike so many of the other Afghan children who are surviving on the streets of Paris.

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Between ages 3 and 5 "the airway is still small but the tonsils and adenoids are at their largest, " says Jodi Mindell associate director of the Sleep Center at the Children's Hospital of Philadelphia.

WSJ: More Children With Sleep Problems Seek Overnight Tests

Dr Rachel Moon, of the Children's Medical Center, in Washington DC, and one of the authors, suggested that children who sleep on the backs at home may be at increased risk of sudden infant death if they are placed on their front during the day.

BBC: Childminders 'raise cot death risks'