A new study has found that lack of sleep and exercise, combined with heavy use of computers, television and video games, puts teens at just as high a risk of developing serious mental health disorders as do risky activities such as binging on alcohol and drugs, according to Time.

The study, one of several released recently on sleep deprivation, was conducted by Swedish researchers from the Karolinska Institute and published in the February edition of World Psychiatry. Researchers called the sleep-deprived teens “invisible" because parents and teachers may not realize that these behaviors are associated with mental illness.

After following nearly 13,000 adolescents in 11 European countries, the study revealed that almost a third were “at greater risk of falling through the cracks,” researcher Vladimir Carli of the Karolinska Institute told Time.

These "invisible" teens “showed similar levels of suicidal thoughts, anxiety, subthreshold depression and depression as adolescents in the ‘high-risk’ group,” according to a report on the study in Medical News Today.

The report identified three main factors that put "invisible" teens at high risk, including spending at least five hours each day watching television, playing (not working) on the Internet and playing video games; sleeping far less (six or fewer hours each night) than recommended; and having a sedentary routine that avoids activities such as sports.

“It is likely that adults won’t see these behaviors as risky or as reason for alarm,” Carli told Time. “But the truth is, they are. It doesn’t mean that every teenager who doesn’t get enough sleep or plays too many video games is at risk. But it is something we need to pay close attention to.”

NPR writes about a separate study published this month in the journal Sleep that indicated that teens who get too little sleep are at serious risk of psychiatric disorders, particularly major depressive disorder (MDD), according to researchers at the University of Texas in Houston.

As many as 11 percent of teens develop major depressive disorder (MDD) by the time they’re 18, NPR notes. Sleep-deprived teens are four times more likely to develop MDD than peers who sleep six or more hours per night. The inverse is also true: adolescents who suffer from MDD are four times more likely to have sleep problems. Lead author Robert E. Roberts told NPR, “That’s a pretty strong reciprocal relationship.”

The Huffington Post reports on a third study out this month that used adult twin pairs of the same gender to determine what role heritability plays in the symptoms associated with poor sleep. Twenty-seven percent of participants who slept within the “normal” range of 7-8-9 hours a night reported depressive symptoms. By comparison, 53 percent of participants who slept less than seven hours and 49 percent of those who slept nine or more hours experienced one or more symptoms linked to depression.

The study was conducted by the universities of Washington, Pennsylvania and Texas.

While there is no “blanket recommendation” for how much sleep adults should get each night, the Huffington Post reports that most individuals need between seven and eight hours to feel rested.

NPR recently reported on yet another study on sleep. This study, from the University of Cincinnati, found that “teens who had very warm relationships with their parents — who said they felt supported, for example, and could talk to their folks — tended to sleep better.”

The National Sleep Foundation recommends daily exercise and sleeping in a dark room to maximize sleep efficacy. Falling asleep and rising at the same time each day is one of the most important ways to help optimize regulation of hormone levels, brain activity and other critical functions.

It is also important to avoid use of televisions, phones and other electronics within two hours of lying down to sleep. Many electronics and some lightbulbs emit a blue light that activates the brain in ways that boost attentiveness and disrupt sleep, according to the foundation. It may be wise to use red lights at night and maximize exposure to blue light during the day, according to research from the University of Toronto reported in the Chicago Tribune.