Does it mean you work with focus from 8am to 5pm? And suppose that you're working in a competitive environment and you won't meet the expectation for your job if you don't work a few extra hours?

I entirely agree it's your right, but if you're ambitious and working more can help you to achieve certain goals, it's not unreasonable to work more than the bare minimum. As long as it's a personal choice, I think it's fine.