If your teenager is having trouble the first things to consider are sleeping, eating, hydration, and safety. Unfortunately, so much of the first three are habits long cultivated in the home, and are very difficult to alter once they become problems and are compounded during adolescence. Nevertheless, these are worth taking a long look at.
It is one thing to wring our hands over the poor eating habits and lack of sleep that characterizes the average teenager, it is another to engage them in a deeper look. Often enough things are not what they seem. Take sleep for instance. The everyday is understudied, and sleep, for all the research that has explored it over the last 20 years, is still not seen by many as all that worthy of consideration. The Times today has an interesting item on the subject, some speculating on sleep itself:
http://opinionator.blogs.nytimes.com/2010/04/21/what-is-sleep/
