This is company dependent rather than country. When I was early in my career as a software dev I felt like I could never say no. I would continue to take on additional work which translated to more hours.
Eventually my manager at the time did the best thing he could for me. He asked me into his office for a "talk". Then he told me that he didn't want to lose me because I burned out. He said, "Look, you have to learn to say no to people. If you aren't comfortable doing that then tell them to run it by me; and then I'll say no."
This is the solution. Your manager understood his job, and its impact on yours. Everyone should be so lucky. So far, I have been! At my first job, my boss told me right off not to let everyone who comes by add work to my pile. It's hard to do at first. After all, your first reaction is to be helpful. But after a while you see the benefit of being able to to deflect things to your manager and have him prioritize the workload.
Where I work, the entire culture changed from "let's work together to make everything better" to "you drop everything RIGHT NOW if the boss decides that they have a personal project" when a new one took over.
...and so I will be starting my new job in a couple of weeks.
Eventually my manager at the time did the best thing he could for me. He asked me into his office for a "talk". Then he told me that he didn't want to lose me because I burned out. He said, "Look, you have to learn to say no to people. If you aren't comfortable doing that then tell them to run it by me; and then I'll say no."