1) possible: watching TV, surfing the internet

Impossible: working, exercising, dieting, eating chocolate, cleaning playing computer games.

2) If they are constantly repeated and lack of variety f activities.

3) For example, with frequent use of the phone, we may have inaccessibility, stress and gradual loss of vision.

4) Every repetitive activity coul become addictive. For example, a lot of people, especially youngsters, can't imagine thir life without regular checking news feed in their smartphones.