Is spending lots of time on social media good or bad for your mental health? Mostly bad, according to a comprehensive new study by the Royal Society for Public Health in the U.K. The Society looked into many different aspects of social media's effect, especially on those between the ages of 14 and 24.

The study found that spending time on social media can be beneficial in some ways, such as helping those with physical or mental health issues find useful information and connect with others facing similar problems. It can also help users find emotional support, a community, and greater opportunities for self-expression.

But these benefits don't make up for the worst effects spending too much time on social media can have on young people. So the Society asked about 1,500 14-to-24-year-olds across the U.K. about the effect that the five most popular social platforms had on their lives, both good and bad, on 14 different measurements, ranging from emotional support to anxiety to cyber-bullying.

Here's some of what they found:

1. Instagram and Snapchat have the greatest negative effect on young people's emotional health.

Of the five most popular social platforms, only YouTube was seen as having a net positive effective on young people's lives, according to survey responses. Instagram, on the other hand, had the most negative net effect, followed by Snapchat. It's easy to see why. Negative body image is a huge risk for young people, especially girls, all of the time--but that risk is intensified on social media. It makes sense that young people faced with a stream of (often enhanced) photographs of others will likely compare their own bodies with the ones in the photos and find fault with themselves by comparison.

A second big issue for young people on social media is fear of missing out, or FOMO, something that can grip adults too, but may be especially dangerous for young people. Here, too, it's easy to see why a photo stream could be a big problem: People often post messages or links to Facebook or Twitter from the comfort of their sofas, but are more likely to post photos when they're out doing something fun.

2. Spending more than two hours a day on social media is a seriously bad idea.

How much is too much? The Society cites research showing that young people who spend more than two hours a day on social media sites are likelier to report mental health issues. Here again, the problem is being constantly subjected to reading about other peoples' lives that seem more fun, more interesting, or more significant than your own.

3. Social media can mean sleep deprivation--which is bad for your brain.

"The brain is not fully developed until a person is well into their twenties and thirties," the study notes. That means sleep deprivation is singularly unhealthy for young people--but frankly it's a really bad idea at every age.

The problem here is that social media can be deeply addictive--possibly more so than nicotine, the study says. And some survey respondents report spending late-night hours scrolling through their social-media feeds when they know they should be asleep.

4. Cyber-bullying is a worse problem than you think.

Seventy percent of young people in the survey say they've experienced cyber-bullying, with 37 percent saying they've experienced it "on a high-frequency basis." In this instance, Facebook is the worst of the social-media platforms, with young people reporting they are twice as likely to be bullied there as on any other social network.

Every major social network has a firm anti-bullying policy, which is good. Unfortunately, the study notes, a recent survey by a British anti-bullying group found that 91 percent of young people who reported being bullied said the social network did nothing to address the problem.

In other words, the problem of cyber-bullying on social platforms is largely up to parents and teachers to solve. That's just one more reason it's important for parents to know how much time their kids are spending on which social networks, and what their experience is like while they're there.