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A pair of economics professors at Brigham Young University have some surprising news for the parents whose teenagers burn the proverbial candle at two ends — the kids are all right.

Well, probably all right, as far as their research goes.

Eric Eide and Mark Showalter published their research, which defies conventional wisdom about the sleep needs of older children, in the Eastern Economic Journal.

"We approached [the issue] as 'Is there any optimal amount of sleep?' " using test scores as a measure, says Showalter. "Our results were very surprising."

Most sleep guidelines in would-be parenting manuals, health texts and pamphlets in pediatrician waiting rooms say that teenagers need 9.25 hours of sleep every night to function well emotionally and academically. That figure seems to have come out of a landmark study at Stanford University sleep labs in the 1970s, the BYU researchers say.

But Eide and Showalter are skeptical of that standard because the teen subjects were put in bed from 10 p.m. to 10 a.m. without distractions. Eide compares it to calculating teens' caloric input by letting them eat all they want over 12 hours.

"If you just let kids sleep as much as the want without any stimuli, apparently they sleep about nine hours," he says.

But the BYU study used academic performance on standardized tests indexed against the number of hours the teens themselves reported sleeping. From that, Eide and Showalter's research indicates seven hours — two hours under the so-called national norm — is the "optimum" amount of sleep for a 16-year-old to score well on tests. In short, they say, the top-performing students are not sleeping nine hours a night.

Here are some guidelines (with an emphasis on guidelines) to optimum sleep, based on the study:

• 9 to 9.5 hours for 10- to 11-year-olds

• 8 to 8.5 hours for 12- to 15-year-olds

• 7 hours for 16- to 18-year-olds

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Too much sleep? • Which brings up another issue: Why would kids getting more sleep score worse? The researchers aren't sure, but they warn not to jump to conclusions. In short, don't wake your teens up if they sleep more than seven hours. But you might want to look into other issues.

"We can't say exactly why we see the decline in scores with increased sleep, like over 10 hours," says Showalter. But medical literature holds some clues — depressed or anxious teens tend to sleep more, and their emotional/psychological issues, not the extra sleep, likely hurt their scholastic performance.

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It's complicated • The findings of the BYU study, as with any study on teenage behavior, must be approached with common sense.

For example, any "average" recommendation, including Eide and Showalter's seven hours, "masks a huge amount of variation," Eide says. As any parent of teenagers can tell you, they make snowflakes look uniform.

Eide offers a take-away from the study: The "9.25 hours of sleep" requirement is likely overblown.

"When parents see these national guidelines, they worry that their teens aren't getting enough sleep." So, relax.

"We found that with teenagers in particular, an optimum of seven hours is about right," he says. "The test scores don't drop off between seven to nine hours. But over nine hours they do."

Eide is willing to stick his neck way out by comparing his research to his own active teens. "They average about seven, eight hours of sleep [per night]," he says. "They fall into the range [indicated as optimum by the study]. They almost never sleep nine hours."

The researchers hope to extend and refine their research by bringing in a key factor in teen sleep patterns — their parents — by keeping careful records on when teens sleep, the quality of their sleep and other variables.

"It all raises the role of parental influences on child sleep patterns," says Eide. "We are examining time-diary data. It's very interesting."