Prior to the internet, a typical child's day would have been school and playtime, dinner with family, maybe a movie or game and then bed. But today your phone, tablet or computer commandeers much of your time, even into the night. You're short of sleep, short of family time, short of face-to-face interactions with the important people in your life, short of vitamin D from being outside which helps our moods.
The internet is NOT a bad place. But just like anything, there are risks. If you eat a donut, it's delicious! But if you eat too many, it will make you sluggish, sick and miserable. Every good thing in moderation.
Most teenagers (and parents) can attest that even too much family time can make a person crazy. Too much of anything can be a bad thing.