Studies have linked habitual sleep loss to weight gain, but new research suggests that this link may depend on how sleep is measured.

In a study of more than 1,500 10- to 19-year-olds, researchers found that sleep, when measured in one way, was related to body weight. Measured a different way, however, it was not.

The findings, which are published in the journal of Pediatrics, do not mean that previous studies linking sleep loss to excess pounds are wrong. But they do suggest that the measurements researchers use to gauge sleep deserve closer scrutiny, the study authors say.

Specifically, their study found that there was a relationship between adolescents’ weight and their self-reported sleep habits - gauged by the question, “How many hours of sleep do you usually get a night?”

Children and teens who said they typically slept for more than 9 hours were less likely to be overweight than their peers who slept for 7 to 9 hours.

But when the study participants were asked to keep detailed diaries on their activities over two 24-hour periods, there turned out to be no link between sleep and weight.

“Our study shows that there is more work to be done in understanding the association between sleep and weight,” said study co-author Dr. Kristen L. Knutson of the University of Chicago.

It does not, however, discount past research showing that sleep deprivation may spur weight gain, Knutson told Reuters Health.

A number of studies have found that adults and children who get relatively little sleep each night are more likely to be overweight than their peers who catch more Z’s. More importantly, Knutson pointed out, controlled sleep-lab studies, where sleep was measured objectively, have suggested that sleep loss can alter hormones involved in appetite and metabolism.

In one small study, healthy young men who slept for only 4 hours for two nights in a row showed a dip in the appetite-suppressing hormone leptin, and an increase in the appetite-stimulating hormone ghrelin.

There’s also evidence that sleep loss impairs normal blood sugar metabolism, Knutson said.

Still, the current findings suggest that more attention should go toward how sleep is measured in studies, according to Knutson. For example, she said, asking some people how much time they spend in bed will accurately measure how much sleep they get; but for people with poor sleep quality, much of that time may be spent tossing and turning.

It might be helpful, according to Knutson, for studies to measure sleep in multiple ways, particularly in objective ways. One example of this is wrist activity monitoring, where a person wears a watch-like device that tracks his or her movements throughout the day and night.

SOURCE: Pediatrics, May 2007.