The internet can have a negative impact on a person's mental health in a few different ways. First, excessive internet use can negatively affect a person's mental health if it takes time away from important offline activities such as sleeping, exercising, school attendance, work performance, and/or opportunities to socialize and interact with others offline.
The internet puts news at our fingertips 24/7, which, for some people, can exacerbate anxiety, especially when the news is less-than-positive. When we are constantly surrounded by worrisome news, it's difficult to stay in a good frame of mind. In addition, some websites, especially social media platforms, make it easy to compare ourselves to other people.
This can really impact our self-esteem and make us worried that we are missing out on life events. When we see other people posting about traveling, attending events, and generally having a great time, we can't help but compare our lives to theirs. But the truth is, they are showing off their "best-of," and it's likely that they have struggles just like we all do.
Lastly, if a person's primary activity on the internet is to gamble (i.e., play online poker, bet on sports, or other casino-type games), they could be prone to becoming addicted.