That's why, Rushton says, it's important for teens and their family members to recognize that long-term depression is not just another phase of adolescence.

"Parents often fall into the trap of just assuming that these behaviors are normal for adolescents," he says. "If they talk with their child's physician, mental health provider or school counselor to find out how the teen is acting and functioning in school or other activities, it will help them determine if their mood is something that's persistent and a possibly serious problem."

According to the NIMH, some of the most common signs that a teen is seriously depressed include ongoing sadness, feelings of hopelessness, anxiety, lack of energy, difficulty concentrating, sleep problems, outbursts, irritability, lost of interest in everyday activities, reckless behavior, social isolation, alcohol or substance abuse, and attempts to run away from home.

Although there is no one single cause of depression, there are several factors that can increase the chance of depression in adolescents. Biochemical studies have shown that one contributor is brain structure development. Rushton says that during adolescence, the brain continues to and be fine-tuned, resulting in different processing of moods and emotions. Even social stressors, like school, a family death and problems at home or with a relationship can trigger a depressed state in vulnerable teens.

Young women are two times more likely than males to not only experience depression, but also to have other difficulties manifest from this condition that can put their health at risk, says Ruston.

Many depressed teens become involved with narcotics, alcohol and cigarette abuse. Also, serious anxiety disorders, social and relationship phobias can be associated with depression.

Since depression may have a genetic component and family mental illness can alter social dynamics and parenting, Rushton recommends that parents consider their family's medical history to be aware of any possible problems associated with mental health that may affect their child.

However, the factors contributing to it can vary with each person and treatment must be individualized. Certain individual treatment plans may involve medication, such as antidepressants, counseling or a combination of the two.

Teens can receive treatment from a variety of mental health specialists - pediatricians, adolescent medical specialists, school counselors, or psychologists or psychotherapists in some cases.

"It's a good start for a teen to talk about their problems in a school or community setting where they feel comfortable and have a good rapport with the people there," says Rushton.

It's also important for a teen to have open lines of communication with parents and other family members to help them deal with serious depression as soon as possible.

"Parents need to stay involved with their teen's school, have frequent teacher conferences and discuss any school issues with their teenager to be aware of any potential problems," says Ruston. "It's also important to have shared activities to foster communication with their teens and also help them through the depressed moods they may experience."

If a parent is concerned about their teen's mental health or are having difficulty talking with their teen about certain problems they are having, Rushton suggests the family contact a mental health care provider or the teen's pediatrician to discuss possible solutions or to receive treatment for the condition if it's necessary.

Facts about teens and depression:

• Depression is a persistent, recurring condition rooted in the brain's chemistry that can last for weeks or months
• Depression will interfere with a person's ability to sleep, eat, concentrate or feel any pleasure in life.
• One in 12 American teenagers experience serious depression, which is often mistaken as a normal stage of adolescence.
• Depression can have a negative effect on a teen's school performance and social activities, and some may get involved with substance or alcohol abuse, or even become suicidal.
• Some of the most common signs of depression to look for in teens are ongoing sadness, feelings of hopelessness, anxiety, lack of energy, difficulty concentrating, sleep problems, outbursts, irritability, loss of interest in everyday activities, reckless behavior, social isolation, alcohol or substance abuse and attempts to run away from home.
• Teens need to have open lines of communication with family members so parents can spot the signs of depression and help their teen receive the treatment he or she needs.