Maintaining sustained productivity and enthusiasm There are researchers who work each day, every day of the year from the moment they reach their office till late. As a young and upcoming researcher, I find that I need breaks after a few days of this kind of heavy working or else I have a mild "crash" following which I get back on the productive track.

My question is: how do these researchers continue to work with no breaks and remain creative and productive? Are there concrete tips that researchers use to ensure they don't keep having periodic crashes and burn-outs?
I think you have some imaginary researcher in mind. They may seem to be working but without any productivity. They may take "breaks" by moving from task to task, say working on a problem, getting stuck, working on a lecture, thinking about family, back to the research problem, etc. 

I think there is research that suggests that breaks aid productivity. I've found that active breaks, are very useful when creativity lags. Bike ride. Then back to work. Aerobic things are especially good. 

Taking regular breaks or "breaks when needed" can actually help you avoid burn out. 

But, more important, when your mind gets into a pattern of ineffective thought, hammering at the same problem over and over is less likely to be effective than putting it down for a bit. Even a nap can sometimes break a log-jam. 

My advice is to not try to work past your natural limits. When you feel the need for a break, take one, even if it is only to walk around the room for a few minutes. Even stretching exercises. Anything to get your mind out of the rut that suggested the break was needed. 

Don't over glamorize work. Probably you are misinterpreting what you are seeing in others. 

On the other hand, many people can (occasionally) get into a groove and are very productive for several hours running. There is a flow and you just ride the wave. I used to do that when programming. But you can't schedule it. Serendipity. It comes. It goes.