"I'm online from the moment I wake up until I go to sleep, whenever I have time -- even in class," the 18-year-old, who gave only her first name, told AFP.
"I'm always messaging friends on 'Line', even when I'm in the bath. I guess I feel lonely if I'm not online, sort of disconnected," she said, referring to a Japanese chat app used by about 90 percent of high school students here, according to a recent survey.
While Sumire acknowledges that she probably uses her iPhone too much, she is far from alone in a country where young people are frequently glued to a screen.
High school girls in Japan spend an average of seven hours a day on their mobile phones, a survey by information security firm Digital Arts revealed this week, with nearly 10 percent of them putting in at least 15 hours. Boys of the same age average just over four hours mobile phone use a day, the research found.
The problem has become so grave a whole field of medicine has developed to ween them off their digital props.
"This is what we call the conformity type," psychiatrist and leading net addiction specialist Takashi Sumioka said. "This type of obsession is caused by the fear that they will get left out or bullied in a group if they don't reply quickly."