It depends, productivity is measured by product output and more time in generally means more product out.   However after a point the returns, because of fatigue, are diminishing and this traditional measurement doesn’t take into account quality which declines as fatigue increases.    For those that work excessively, and the concept of “excessive” can very largely individual to individual, they may actually have less high quality results than others who better manage their work and leisure.

The more creative the task, it seems, the more critical rest seems to be, the more repetitive and instrumented (the less thinking that is involved), the less critical the requirement but even here injuries tend to increase with fatigue and injuries clearly adversely impact productivity.  

So the key isn’t working less for everyone, but in working optimally and optimally means balancing work and leisure so that high quality results are optimized.