Sleep and grow lean - if you are a teenage boy, that is

The less teenage boys sleep, the more body fat they have - but the same is not true for teenage girls, an Otago University study has found.

The research looked at the sleeping patterns and height, weight and fat ratios of hundreds of Otago teenagers aged 15 to 18.

Lead researcher Paula Skidmore, from the university's department of human nutrition, said she found significant results linking sleep duration and body fat in boys, but not girls.

For an average 16-year-old boy weighing 69.5kg and standing 1.76m tall, an extra two hours sleep a day made a 9% difference in body fat.

The study found boys who got eight hours sleep had 1.6kg more body fat and a 1.8cm bigger waist than boys who slept 10 hours. It also found boys who slept eight hours had 1.8kg more lean mass, or bone and muscle mass, compared with boys who slept 10 hours - an increase of 1.4%.

Girls who slept eight hours a day showed no discernible change in body-fat composition or waist size, compared with girls who slept 10 hours a day.

Dr Skidmore said the key to the research, which drew on results from 386 boys and 299 girls from Otago, was that it looked at adolescents.

''Most of the research in this area to date has focused on younger children, whose parents tend to be stricter about bedtimes. Teenagers get more leeway over bedtimes.

''Our results suggest that for older teenage boys, making sure that they get adequate sleep may help to maintain a healthier body. It seems to be that, within reason, the more sleep the better for boys.

''It was unexpected that we did not find the same result in girls, who may actually be more aware of their diet and more in tune with a healthier lifestyle.''

The study ruled out out the effects of food choice and the number of screens, such as TVs and consoles, the teenagers had in their bedrooms.

Overall, it found 19% of the boys and 22% of girls were overweight, and 8% of boys and 6% of girls were obese.

Both boys and girls slept on average for around nine hours 15 minutes a day, with a quarter of teens sleeping for less than eight and a-half hours, and another quarter sleeping for more than nine hours 45 minutes.

Both boys and girls tended to sleep for an hour more at weekends than on weekdays.

The researchers used fat mass index as well as body mass index (BMI) for their calculations. The former is a measure of body fat, whereas BMI measures excess weight, not necessarily excess fat, particularly in overweight adolescent boys.

Dr Skidmore said they found a link between BMI and sleep but not as strong as the relationship with fat mass.

''We think that in this group of growing adolescents that more direct measures of body fat are needed to pick up the subtle changes in your body when you are growing.''

The study was published in the UK's Nutrition Journal.

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