Sitting for too long is never a good idea. Alternating your activities helps to break up long periods of sitting down: stand up or go for a short walk after every 30 minutes of uninterrupted sitting. Besides ensuring you spend less time sitting down, this has a positive effect on your physical health and mental well-being.
Active, dynamic sitting also helps if you have a sedentary job. For this, you can use a chair that has a movement mechanism, which constantly obliges you to shift your centre of gravity. A Löffler chair, for instance. Dynamic sitting is important to maintain a strong supply of blood to your muscles and prevent problems. Even so, you still need to get up and walk around regularly.