The majority of preschoolers may not be getting the amount of sleep they need each night, placing them at higher risk of being overweight or obese within a year, according to a new study.

Published online by the journal Sleep Medicine, the study investigated links between mothers’ employment status and their children’s weight over time, exploring the impact of potential mediators, such as children’s sleep and dietary habits, the amount of time they spent watching TV, and family mealtime routines.

“The only factor of the four that we investigated that mediated the relationship between maternal employment status and child obesity was how much sleep the child was getting each night,” says lead author Katherine E. Speirs, a postdoctoral research associate in human and community development at the University of Illinois, in a release.

Speirs and co-authors Janet M. Liechty and Chi-Fang Wu for 1 year followed 247 mother-child pairs from the STRONG Kids study. A health awareness initiative for families that focuses on preventing child obesity, the study is coordinated by the university’s Family Resiliency Center.

The children, who ranged from 3 to 5 years old, were weighed and measured, and had their body mass index calculated, at the outset of the study and again 1 year later.

At the second weigh-in, 17% of the preschoolers were overweight and 12% were obese, according to BMI-for-age growth charts.

Of the mothers in the sample, 66% were employed full time, defined as working 35 hours or more per week. Another 18% of the women were employed part time, or 20 to 34 hours per week.

Children whose mothers worked full time got fewer hours of sleep than peers whose mothers worked less than 20 hours per week. The children of women who worked full time also tended to have higher BMIs at the second weigh-in.

Just 18% of the preschoolers in the sample were getting the 11 to 12 hours of nightly sleep recommended by the National Heart, Lung and Blood Institute, the researchers found.

On average, the children were getting about 9.6 hours of nighttime sleep. Each additional hour of nighttime sleep that a child obtained was associated with a 6.8% decrease in their BMI at the second weigh-in, the researchers found.

“We looked at nighttime sleep in particular, because studies show that the amount of nighttime sleep matters for regulating weight,” says Liechty, a professor of medicine and of social work.

“We think that it might be the more hours that mothers are working, the less time they have, and there may be some sort of trade-off going on, ‘Do I spend quality time with my child or do we get to bed early?’” Speirs says. “And then in the morning, when mothers leave for work, their children also wake up early to get to day care.”

Mothers whose children were enrolled in 32 licensed day care centers in Central Illinois were recruited for the study: 66% of the women had college degrees; about a third had household incomes under $40,000 a year, and just over half the sample had household incomes under $70,000 a year.

“The challenges of ensuring that children obtain adequate sleep may be even greater for low-income women, who often hold multiple jobs or work rotating shifts or nonstandard hours,” Speirs says.

“There are lots of characteristics about mothers’ employment that are really important to help us better understand the relationship between mothers’ employment status and child obesity, such as whether women are working part time voluntarily or involuntarily, or scheduled or nonscheduled hours,” said Wu, a professor of social work.

The authors are exploring some of these characteristics and possible links with child obesity in a related study, which is underway.