Letting children stay up past 9pm more than doubles their chances of becoming obese by the time they are a teenager, a study claims.
The benefits of an early night for children extended far beyond stopping them being cranky, the researchers said.
Preschool-aged children whose bedtimes were 8pm or earlier were less than half as likely to be obese ten years later than those allowed to stay up past 9pm.
One theory is that when parents set strict rules about bedtime their children get used to routines in their own lives. People with good sleep habits tend to be healthier and do more exercise.
Ohio State University looked at data from an ongoing nationwide 25-year project called the Study of Early Child Care and Youth Development. The researchers analysed more than 1,300 children who were born in 1991 from different cities and different backgrounds.