(do something) for long periods

Doing something for "long periods" means that you do it for a long time without stopping.

For example, if you have to stand up at work and don't get a chance to sit down for a few hours, you can say that you "stand for long periods" at your job.

(something) is bad for your health

If something makes you unhealthy, it's "bad for your health". Use this especially when you're talking about habits and things that you do often:

Chronic stress is bad for your health.

A recent study shows that sitting for long periods of time could be bad for your health.

A recent study shows that (clause).

This is how scientific studies are often reported in the news.

A "study" is a research project done by scientists or social scientists. We talk about the results of the study by saying what it "shows":

Our research shows that there may actually be liquid water present beneath  the moon's surface.

Studies have shown that women who take folic acid supplements during early pregnancy reduce the risk that their baby will be born with neural tube defects.