Right I have found the latest study
"Spending more than 40 hours a week in childcare can worsen behaviour in young children, according to a new report.
The latest report from the Growing Up In Scotland longitudinal research, which is following the lives of 5,000 babies and 3,000 toddlers throughout childhood, found that placing children aged 34 months in childcare for long periods had negative effects on their behaviour. The effect was most significant in girls and in children whose mothers were aged under 25 when their child was born."
This is referring to an earlier study
"The bad behaviour of children who have spent long hours in nursery care rubs off on their classmates when they start school, say researchers.
Gordon Brown's childcare research chief has already warned that toddlers who spend long hours in nurseries or with childminders suffer "disconcerting" effects.
Professor Jay Belsky said these include difficult relationships with their mothers and aggressive and disobedient behaviour when they start school."
and an earlier study
"The research published by the DfES found that children spending more than 35 hours per week in a nursery were more likely to be "anti-social, worried and upset"."
"The research - led by academics at Oxford University and other leading research centres - came as teachers warned that young babies risked being ?institutionalised? by the state because more mothers were being encouraged to return to work."
so it seems the experts can't agree on what is meant by long hours either but it seems they mean between 35 and 40 hours