The children of mothers who worked nonstandard schedules performed much worse on tests evaluating such things as language development, memory, learning and problem solving, along with knowledge of colors, letters, numbers and shapes.

Professor Han's findings, published in the January/February 2005 issue of Child Development, come as an increasing number of mothers entering the workforce are finding employment with nonstandard hours.

Single, low-income mothers, as well as welfare mothers, are more likely to work such nonstandard hours, Dr. Han notes, and they often have difficulties balancing work and family responsibilities on such schedules, according to earlier studies.

Based on Professor Han's study, it appears those difficulties can have an adverse impact on their children's cognitive development.

Cognitive Performance 'Much Worse'

Professor Han used information from the National Institute of Child Health Development's (NICHD) Study of Early Child Care, which tracked 1,364 children from 10 sites around the country from birth in 1991 through 36 months.

She focused on 900 children whose mothers had worked in the first three years of their child's life. About half of them worked nonstandard hours during this time.

Even after controlling for the quality of the home environment and child care, maternal depression, and the mother's sensitivity toward her children, Professor Han found that the children of mothers who worked nonstandard schedules during their first three years of life performed much worse on cognitive tests.

Those tests evaluated such things as language development, memory, learning and problem solving, along with knowledge of colors, letters, numbers and shapes.

Missing Out on School Preparation?

One explanation for the effect, Professor Han suggests, may lie in the type of care children receive when their mothers work such hours.

Earlier studies found that mothers working nonstandard hours were less likely to put their children in center-based child care than mothers who worked more traditional hours. Such child care has been linked with better cognitive outcomes in children.

"The children of mothers working at nonstandard hours may miss out on this opportunity to get what could be a form of school preparation that many other children receive," notes Dr. Han.