Lights out, phones on. Many teenagers reach for their smartphones or tablets at bedtime, but it may be a good idea to switch off before you hit the pillow.

    According to a study by researchers at Murdoch and Griffith Universities in Australia, using mobile devices late at night could harm teenagers' sleep and lead to an increased risk of mental health problems.

    The study was carried out on more than 1,000 Australian high school students aged between 13 and 16. It lasted four years as student participants moved from Year 8 to Year 11.

    During the study, students were asked what time of the night they received or sent text messages and phone calls, and how well they slept.

    The results are impressive. When the study started in Year 8, more than 85 percent of the students had a mobile pone and around one-third of these students never used phones after lights out. In contrast, 93 percent of Year 11 students had a mobile phone and only 22 percent of then had no late night mobile use when the study concluded four years later.

    As the levels of mobile phone use increased, there was a growth in poor sleep behavior among the students. Meanwhile, mental health problems also increased. Students in Year 8 who were more likely to use phones at night were found to have higher levels of depression (抑郁) and lower self-esteem (自尊心) one year later.

    Lynette Vernon, lead author of the study, explained that phones affect our sleep patterns because of the unnatural blue light they produce. This light could affect the level of hormones (荷尔蒙) in the body, disrupting (打乱) its balance.

     “The more you sit in bed and watch TV, play with your tablet, and play with your phone at three o'clock in the morning, the more these things affect the slow waves of your sleep pattern and give your poor quality sleep,” Mark Levi, a Sydney-based sleep doctor said.

    Based on the findings, the researchers suggested parents should focus on guiding their children's use of mobile devices.

    According to Vernon, the solution is not simply to ban teenagers from using their devices, but teach them better ways of using them

     “Back when they're aged 7 to 10, you have to be a role model. If you put your phone away at night and it doesn't go into your bedroom, it becomes normalizedin the household and you have a much easier job,” he told The Guardian.