It's the mystery of mysteries -- especially to parents -- the unpredictable and sometimes incomprehensible moods and behaviors of the American teenager. Generations of adults have pondered its cause. Hormones? Rock music? Boredom? Drugs?

In "Inside the Teenage Brain," FRONTLINE chronicles how scientists are exploring the recesses of the brain and finding some new explanations for why adolescents behave the way they do. These discoveries could change the way we parent, teach, or perhaps even understand our teenagers.

New neuroscience research has shown that a crucial part of the brain undergoes extensive changes during puberty -- precisely the time when the raging hormones often blamed for teen behavior begin to wreak havoc. It's long been known that the architecture of the brain is largely set in place during the first few years of life. But with the aid of new technologies such as magnetic resonance imaging (MRI), scientists are mapping changes in pre-teen and teenage brains and finding evidence that remarkable growth and change continue for decades.

The vast majority of brain development occurs in two basic stages: growth spurts and pruning. In utero and throughout the first several months of life, the human brain grows at a rapid and dramatic pace, producing millions of brain cells.

"This is a process that we knew happened in the womb, maybe even in the first 18 months of life," explains neuroscientist Dr. Jay Giedd at the National Institute of Mental Health. "But it was only when we started following the same children by scanning their brains at two-year intervals that we detected a second wave of overproduction."

This second wave -- occurring roughly between ages 10 and 13 -- is quickly followed by a process in which the brain prunes and organizes its neural pathways. "In many ways, it's the most tumultuous time of brain development since coming out of the womb," says Giedd.

Confronted by these new discoveries, academics, counselors, and scientists are divided on just what all this means for children.

"Our leading hypothesis ... is the 'use it or lose it' principle," Jay Giedd tells FRONTLINE. "If a teen is doing music or sports or academics, those are the cells and connections that will be hardwired. If they're lying on the couch or playing video games or [watching] MTV, those are the cells and connections that are going to survive."

But others voice caution in leaping to conclusions about the implications of these findings.

"The relationship between desired behaviors and brain structure is totally unknown," John Bruer tells FRONTLINE. He is president of the James S. McDonnell Foundation and author of The Myth of the First Three Years. "This simple, popular, newsweekly-magazine idea that adolescents are difficult because their frontal lobes aren't mature is one we should be very cautious of."

This FRONTLINE report also looks at research that is helping scientists understand another puzzling aspect of adolescent behavior -- sleep.

Mary Carskadon, director of the E.P. Bradley Hospital Sleep Research Laboratory at Brown University, has spent years mapping the brains of sleepy teens. She has calculated that most teens get about seven and a half hours of sleep each night, while they need more than nine. Some say these sleep debts can have a powerful effect on a teen's ability to learn and retain new material -- especially abstract concepts like physics, math, and calculus.

Despite all the new scientific research, "Inside the Teenage Brain" suggests that there is a consensus among experts that the most beneficial thing for teenagers is good relationships with their parents. Even Dr. Giedd wonders about the kinds of lessons parents can draw from his science. "The more technical and more advanced the science becomes, often the more it leads us back to some very basic tenets. ... With all the science and with all the advances, the best advice we can give is things that our grandmother could have told us generations ago: to spend loving, quality time with our children."

Ellen Galinsky, a social scientist and the president of the Families and Work Institute, has seen scientific fads come and go. But she says her research for a book about children shows there are enduring lessons for parents. Drawing on her interviews with more than a thousand children, she found that, to her surprise, teens were yearning for more time and more communication with their parents, even when they seemed to be pushing them away. She told FRONTLINE, "Even though the public perception is about building bigger and better brains, what the research shows is that it's the relationships, it's the connections, it's the people in children's lives who make the biggest difference."