Researchers in Texas and Sweden report that teens who don’t get enough sleep are four times more likely to develop depression than those who do. The circadian rhythm changes during puberty, making it difficult for teenagers to catch enough Z’s. Seventy percent of high schoolers don’t get the recommended 9-10 hours. Surprisingly, the research shows that sleep shortage, lack of exercise and excessive Internet use are as likely to cause depression as drug misuse and truancy. Teens now have the perfect retort to accusations of laziness — sleeping in is medically advised.