Working online means you will be sitting on one place for many hours and you will be on your computer for long hours. Sitting on one position for long hours and using computers for a long time will have adverse effect on your physical and mental health. 
Here are some health tips for online workers
Do not work straight for 8 hours long. Work in short sessions. When you work in short sessions you can take breaks for stretching your body. 
Drink a lot of water. When you drink water, you will have to frequently rush to the bathroom. This will also allow you to stretch your body. 
Do not munch junk food. If you are in habit of eating while working, eat healthy snacks. 
Have green plants in your room. Look at the green plants once in a while. Green plants will relax your eyes.