It's common now for people to be sitting down for most of the day. They say "sitting is the new smoking", and to an extent, this statement has some truth to it. Sitting for long periods of time is simply unhealthy. Having said that, there are many ways to mitigate the health problems created by sitting. This is not an exhaustive list, but it should highlight some basic issues.
Ensure the screen height is at eye level
In most jobs, sitting down usually involves sitting in front of a screen. The potential problem here is that typically screens are too low down, so people are bending their neck to focus on the screen. Over time, this can cause real problems with the vertebrae in the neck and muscles and nerves surrounding the bone. Neck pain can ensue (known as "tech neck" because it's caused by sitting in front of a computer). You can also get back pain too, since if you're stooping over the screen, your back is likely to be straining too. By ensuring the screen is at eye level you neck is straight, and so too is your back.
Wear computer glasses
filter blue light and screen glare. This greatly reduces the strain and fatigue your eyes feel from staring at a screen for prolong periods.
Get out of the chair for at least five minutes per hour
That's right - get out of your chair and walk around your office or home every hour for at least five minutes. Stretch, look out the window, focus on varying distances. Most people will complain "but it takes me a while to get back into the work...it's inconvenient!". Make a habit of it - you will find yourself looking forward to these breaks as they invigorate you.
Exercise outside of work time
This is a biggie right here. Go swimming, cycling...do some cardio that gets the blood moving and heart working harder. Your body is there to be used and it's not designed to sit down all day. Give it a workout, and you'll feel a lot better for it, while "earning" your sleep at night.