Many tired people keen to get a better night's sleep focus on changing habits before bedtime but a new study shows the key to restful slumber lies in how someone spends their day.

A University of South Australia study of more than 2500 children and their parents found those who did moderate to vigorous physical activity during the day had better shut-eye.

They felt less tired and had better quality sleep, according to researcher Dr Lisa Matricciani.

"When people think about sleep quality, they tend to focus on adjustments immediately before bedtime," Dr Matricciani said.

"For example, avoiding screens, not eating too much, and avoiding alcohol – but our research looks beyond this to the range of activities we undertake during the day."

Dr Matricciani said while activities like playing video games late into the night directly impact sleep, people also need to think about what they are doing during daylight hours.

"If it's simply a matter of being more active during the day, then it may be a relatively achievable goal for most of us," she added.

School-aged children need between 8-11 hours of sleep per night and adults need between 7-9 hours, according to the Australian Institute of Health and Welfare.