Cortisol, the primary stress hormone in humans, tends to be at its highest levels in the early morning and gradually declines over the course of the day. But recent research has found that many preschoolers in full-day child care have increases in cortisol from morning to afternoon.

This study found that children in classrooms with closer to 10 children were more likely to show cortisol decreases from morning to afternoon, while children in classrooms with closer to 20 children tended to show greater increases in cortisol across the day. Children with more clingy relationships with their teachers showed greater rises in cortisol from morning to afternoon, and children with more conflicted relationships with their teachers showed greater cortisol boosts during a one-on-one session with their teachers. Conflicted relationships were said to occur when teachers tried to control resistant children, when children perceived their teachers as unfriendly, or when teachers or children reported that the teachers found the interaction frustrating.

This unusual increase of cortisol levels is of potential concern because long-term or frequent elevations in cortisol can have negative health consequences. Research with animals and human children suggests that secure relationships with parents protect children from rises in cortisol in stressful situations.

This study, by researchers at Washington State University, Auburn University, the Washington State Department of Early Learning, and the Pennsylvania State University, appears in the November/December 2008 issue of Child Development.

The study looked at 191 preschoolers attending 12 child care centers in a small southeastern U.S. community to determine if the quality of teacher-child relationships could predict increases in cortisol in the children. Teachers described their relationships with the children in their care on a questionnaire and children talked about their relationships with their teachers in interviews. Researchers also collected saliva samples from the children in classrooms to determine changes in their cortisol levels from morning to afternoon. They also collected saliva outside of class before and after a series of mildly difficult tasks designed to look like challenges the children might experience in the classroom and before and after a non-challenging interaction with the teacher.

"This study sheds additional light on an as yet incompletely understood phenomenon¬ among many young children attending full-day child care," according to Jared A. Lisonbee, assistant professor of human development at Washington State University and lead author of the study. "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."