To the researchers' surprise, the data gathered on 105 children showed that far from providing a baseline, cortisol levels were already high three to six months before school started.

Lead researcher Julie Turner-Cobb of the University of Bath wonders if parental stress about their children starting school is being passed on to the children.

The study, published in the Economic and Social Research Council's Society Today, also found children with a more shy, fearful personality appeared less stressed than their more extrovert peers and children with higher levels of cortisol throughout the day when starting school actually had have fewer sick days.

Stress levels lowered significantly after six months, suggesting most children adapt well to school. A few children still had high cortisol levels throughout the day at follow-up, indicating that they were experiencing a more long-term stress response that Turner-Cobb suggests could lead to poorer health.

Turner-Cobb observed these children were more extroverted but had also become increasingly socially isolated during the study -- perhaps because their peers had lost patience with their exuberance.