Pre-school children who are in bed by 8pm every night are significantly less likely to become obese during adolescence compared to children who go to bed even an hour later, a new study has found.

According to the findings, young children who go to bed after 9pm are twice as likely to become obese teenagers.

The US study is based on data from almost 1,000 children who were part of an ongoing study. The researchers divided pre-school bedtimes into three groups - before 8pm, 8-9pm and after 9pm.

The children had an average age of four for the initial part of the study and were followed up when they were teenagers, with an average age of 15.

The researchers found that only around 10% of the children who had had the earliest bedtimes became obese during their teen years compared to 16% of those who had gone to bed between eight and nine. However, when it came to those who had gone to bed after 9pm during their pre-school years, 23% were found to be obese teenagers.

Previous research has suggested a link between shorter sleep duration and obesity and one study did find a link between late bedtimes and obesity risk five years later. However, this is the first study to use obesity data that had been collected around a decade after children were in pre-school.

"For parents, this reinforces the importance of establishing a bedtime routine. It's something concrete that families can do to lower their child's risk and it's also likely to have positive benefits on behavior and on social, emotional and cognitive development," commented the study's lead author, Sarah Anderson, of Ohio State University.

The study found that later bedtimes were more common in lower-income households and in homes where mothers had less education.

Ms Anderson explained that the researchers focused on bedtimes because they have a greater influence on the duration of overall sleep compared to wake times, which parents often have less control over. For example, if a parent needs to be in work early, young children may have to rise early as well depending on childcare.

She acknowledged that even if a child goes to bed early this does not necessarily mean that they will fall asleep immediately, however she insisted that following a consistent bedtime routine will make it more likely children will get the amount of sleep that they need.

"It's important to recognise that having an early bedtime may be more challenging for some families than for others. Families have many competing demands and there are tradeoffs that get made. For example, if you work late, that can push bedtimes later in the evening," Ms Anderson noted.

However, she pointed to previous research which found that most children are biologically programmed to be ready to fall asleep well before 9pm.

Ms Anderson added that more research is needed to ascertain how sleep works with other factors that can affect weight gain in childhood, such as diet and exercise.

Details of these findings are published in the Journal of Pediatrics.