Can I start writing my PhD thesis from the second year? I'm going to enter my second year in my PhD after two months from now. I start to think that I can write some chapters from now instead of waiting until the fourth year. This would help me to finish my study early is that possible or at least avoid the stress of writing in my fourth year. 

However, most of my friends who are in the third and fourth year suggest not to do that because it is going to waste my time and instead I need to focus more on the literature review and prepare for the field work.

Need advice about this from students who go through this.
It is probably a good idea to keep a running outline of your work as you go. Also, sometimes you get stuck for some period of time - say waiting for experiments to mature. In those periods if you can't otherwise make progress it might be a good idea to write up (and to revise) what you already know. 

I wouldn't do this to "shorten" the time, however, but rather to keep myself on track and to keep an overview of the work in mind. 

One advantage of this, if you have the discipline to do it, is that the thesis gets revised many times this way before you finish, rather than only once or twice. 

In short, it could be time wasted or time well spent, depending on when you do it and how you approach it.