The study included nearly 2,200 kids in 20 US cities. One-third of them had consistent, age-appropriate bedtimes between ages 5 and 9, according to their mothers.
Compared to that group, those who had no bedtime routine at age 9 got less sleep and had a higher body mass index (an estimate of body fat based on height and weight) at age 15, according to the Penn State study, Medical Xpress reported.
"Parenting practices in childhood affect physical health and BMI in the teenage years. Developing a proper routine in childhood is crucial for the future health of the child," study co-author Orfeu Buxton said in a university news release.
Buxton is director of the Sleep, Health and Society Collaboratory at Penn State.
"We think sleep affects physical and mental health, and the ability to learn," he added.
The findings highlight the importance of educating parents about children's bedtimes.
Several factors should determine bedtimes. They include what time the child must get ready for school, how long it takes to get there and the school's start time, according to the researchers.
"Giving children the time frame to get the appropriate amount of sleep is paramount," Buxton said.
Bedtime should be set to give the child an adequate amount of sleep, even if he or she doesn't fall asleep right away, he explained.