In the United States, the number of teenagers who suffered from the classic symptoms of depression rose 33% between 2010 and 2015. During the same period, the number of those aged 13 to 21 who committed suicide rose by 31%. Psychologist Jean Twenge and her colleagues found that this growth of mental health issues was not divided along the lines of socioeconomic class, race/ethnicity, or geographical location. Rather, it was associated with spending more time in front of a screen. In general, suicide risk factors—depression, contemplating, planning, and attempting suicide—increase significantly if the subject spends more than two to three hours online. Especially, those who spent five or more hours had their suicide risk factors increase 71%. It is not clear, however, whether depression causes a teenager to spend more time online or the other way around. At the same time, teens who spent more time online were more likely to not have enough sleep, a major predictor of depression. Many teenagers told researchers they used a smartphone or a tablet right before bed, kept the device close, and used it as an alarm clock. But the blue light emitted by these devices, texting, and social networking are known for perturbing sleep. Besides mental problems like depression and anxiety, sleep deprivation is also linked to reduced performance in school and obesity. Parents can address the problem of sleep deprivation simply by imposing limits on screen time and buying simple alarm clocks.