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.
Parents reported their children's bedtime routine aged four. When the children were 15 their height and weight were then measured to estimate their Body Mass Index.
They found 23 per cent of children who had gone to bed after 9pm were obese ten years later, compared with 10 per cent of those regularly in bed before 8pm.
Lead researcher Sarah Anderson said: "It's something concrete that families can do to lower their child's risk and it's also likely to have positive benefits on behaviour and on social, emotional and cognitive development."