The study co-authored by postdoctoral fellow at UniSA, Dr Carol Maher and Professor Tim Olds examined the bedtimes and waking times of more than 2000 Australians aged between 9 and 16 years comparing their activity in their free time and their weight.
The results showed that even with equal actual amounts of sleep, young people who went to bed early and woke early were likely to be slimmer and fitter that their counterpart night owls.
“We found that kids who went to bed late and got up late were 1.5 times more likely to become obese and 2.9 times more likely to be physically inactive,” Dr Maher said.
“The night owls more often spent their free time playing computer or video games, watching TV or engaged in other sedentary or screen-based activities.
“While scientists have already made the connection between less sleep and poor health outcomes around obesity and fitness, what is interesting and new here is that the timing of sleep may be an important factor in predicting health in young people.”
Dr Maher said given teenagers’ natural inclination to stay up and sleep in, the study may help to alert people to the dangers of taking that habit to extremes.
“We know that evenings tend to be the time of day when there are more sedentary activity options,” she said.
“The most attractive TV programming is in the evening and it is a time when people hop onto facebook or socially interactive online gaming options so the incentives are there for teenagers to stay up and stay sedentary. At the same time, when they sleep in they are missing the opportunities for sports and other physical activities that tend to be held or undertaken in the mornings.”
The study showed young people who habitually went to bed early and woke up earlier than their late-sleeping contemporaries accumulated 27 minutes more moderate to vigorous physical activity per day.
Night owls spent an average of 48 minutes longer playing video games, watching TV or engaging online than those who turned in early. In fact, on a broad scale, they replaced 30 minutes of moderate to vigorous activity with 30 minutes of sedentary behaviour each day.
And in the line-up of indicators for poorer health outcomes, the night owls had higher Body Mass Index (BMI) scores and were more likely to be obese or overweight.
The night owls were also more likely to live in major cities, come from lower SES households, work part time and have fewer brothers and sisters.
“It is only when you do the research and unpack the dynamic relationships between health and habits that you find trends that can potentially be altered with modifications to behaviour and the social environment,” Dr Maher said.
“The research may help to support education around teen-age health and give them the knowledge to improve their own health and well being.”