In this digital age, it seems that everyone has a cell phone. But a new study says that cell phones may have hidden dangers: using them after lights-out may be putting teens at increased risk for depression and suicidal thoughts.

The study, from Japanese researchers, examined 18,000 young people in junior and senior high school in Japan. The psychologists asked them questions designed to discover symptoms of anxiety and depression. Questions also attempted to uncover suicidal thoughts and self-harm. Researchers also asked participants how many hours of sleep students received a night and how often they checked emails or talked on the phone after going to bed.

They found that teenagers who used their cell phones after going to bed were more likely to suffer from poor mental health and depression. That link held steady even when researchers controlled for other factors that have been known to affect mental health, like drug and alcohol use.

Additionally, use of the phone after bedtime correlated with fewer hours of sleep. The research was not altogether surprising for many psychologists. A significant amount of research has found that sleep is linked with mental health. A study published last year in the Journal of Psychiatric Research found that teens who had trouble sleeping were at increased risk for suicide and self-harm.

Researchers theorize that looking at a bright screen and performing tasks that excite the brain may change the production of melatonin. The hormone is produced during sleep, and a change in its production could hurt teens' quality of sleep.

Some say that they are unsure about the reasons for nighttime cell phone use. Some children may be reaching out for help while others believe that mental health problems could cause people to stay up late and to use their phones. One psychologist also said that the study did not go far enough, as children and teens can also be distracted by the television, computers, and video games.

In addition, some believe that the findings in Japan may not necessarily apply to Americans. Japan has one of the highest suicide rates in the world. But most psychologists are in favor of leaving cell phones out of the bedroom, though more research needs to be conducted on whether that improves mental health.

The recent study was published in the Journal of Pediatric Psychology.