A study in the Oct. 1 issue of the journal Sleep recorded the bedtimes and wake times of 2,200 Australian participants, ages 9 to 16, and compared their weights and uses of free time over four days. Children who went to bed late and got up late were 1.5 times more likely to become obese than those who went to bed early and got up early. Furthermore, late-nighters were almost twice as likely to be physically inactive and 2.9 times more likely to sit in front of the TV and computer or play video games for more hours than guidelines recommend.

"The children who went to bed late and woke up late, and the children who went to bed early and woke up early got virtually the same amount of sleep in total," said co-author Carol Maher, PhD, a postdoctoral fellow with the University of South Australia. "Scientists have realized in recent years that children who get less sleep tend to do worse on a variety of health outcomes, including the risk of being overweight and obese. Our study suggests that the timing of sleep is even more important."

Maher said mornings are more conducive to physical activity for young people than nights, which offer prime-time TV programming and social networking opportunities. This relationship between time of day and available activities might explain why more sedentary and screen-based behaviors were observed with later bedtimes, she said. At a time when research is showing that teenagers have a natural tendency to stay up late and wake late, the results of this study could stand as a warning.

"It is widely accepted that the sleep patterns of adolescents are fundamentally different from children and adults, and that it is normal for adolescents to stay up very late and sleep in late in the morning," Maher said. "Our findings show that this sleeping pattern is associated with unfavorable activity patterns and health outcomes, and that the adolescents who don't follow this sleep pattern do better."

Other findings from the University of South Australia study:

• Early-bed/early-risers went to bed 70 to 90 minutes earlier, woke up 60 to 80 minutes earlier and accumulated 27 minutes more moderate to vigorous physical activity each day than late-risers.
• Late-bed/late-risers watched TV, played video games or were online 48 minutes longer each day than early-bed/early risers, primarily between 7 p.m. and midnight.
• Only 12 percent of late-bed/late-risers had an average of two hours or less screen time per day, which is recommended for children and teens by the Australian Department of Health and Aging. In comparison, 28 percent of early-bed/early risers met the recommendation for screen time.
• On a broad scale, late-bed/late-risers replaced about 30 minutes of moderate to vigorous physical activity with 30 minutes of sedentary behavior each day, relative to the early-bed/early-rise group.
• Body-mass index (BMI) scores were higher in late-risers than early-risers, and late-risers were more likely to be overweight or obese.
• Late-bed/late-risers tended to have few siblings, live in major cities, come from lower household incomes and have a part-time job.

The Australian Food and Grocery Council contributed financially to this study.

Note: If no author is given, the source is cited instead.

• Sleep Disorder Research
• Healthy Aging
• Insomnia Research

• Sleep Disorders
• Insomnia
• Child Development

• Circadian rhythm sleep disorder
• Delayed sleep phase syndrome
• Separation anxiety disorder
• Night terror
• Rapid eye movement
• Sleep deprivation