Mothers who send their young kids to bed before 8:30pm have better mental health than those who don't.

An early bedtime may lead to healthier children and happier mums.

A study of 3600 Australian children aged four to nine found a bedtime for younger children may be more important than a long sleep.

The children who went to bed before 8:30pm had better health-related quality of life than those children who were late to sleep.

The mothers of the early sleepers also reported better mental health.

"So mums and dads, getting kids to bed early is not just great for them. It's good for you too," said lead researcher Dr Jon Quach.

"These benefits were seen in all early-to-bed kids regardless of whether they woke early or slept late."

Another study has shown benefits of early bedtimes that extend into adolescence.

A University of South Australia study of 2200 children found those aged nine to 16 who went early to bed and got up early were more likely to have smaller waistlines.

"The late sleepers were considerably more likely to be obese, have a poorer diet, get more screen time and less physical activity than other kids," said lead researcher Professor Tim Olds.

Further research was needed to see how much impact bedtime had on weight, he said.