A study says that excessive use of smartphones and tablets may cause sleep deprivation in teens. The study found that adolescents who spend more than two hours a day with their gadgets are more likely to be sleep deprived.<br/><br/>Most sleep experts agree that adolescents need nine hours of sleep each night to be engaged and productive students. The study says that less than seven hours is considered to be insufficient sleep.<br/><br/>The study, published in the journal Sleep Medicine, also found that the more time young people reported spending online, the less sleep they got.