The stress hormone cortisol usually peaks in people in the morning, then decreases through the day.

But researchers have found that some preschoolers' cortisol levels rise through the day when they are in full-day child care.

Children in classes of 10 or less were more likely to show the normal decrease, but those in classes with closer to 20 others tended to show a rise.

The study of 191 preschoolers at 12 places also found that children who were clingier with teachers had more stress. Researchers surveyed students and teachers, and collected saliva samples from the children

"This study sheds additional light on an as-yet incompletely understood phenomenon among many young children attending full-day child care," wrote lead author Jared A. Lisonbee of Washington State University. "Additionally, the study begins to situate child care-cortisol research in the context of a broader literature on the role of relationships in shaping how children function and how they react to stress."

A news release on the study did not indicate what changes in health or behavior could be expected from children with higher cortisol levels.

The study appears in the November/December 2008 issue of Child Development.