More work doesn't need to be done, the same amount needs to be done in 4 days as in 5. Saying this, some careers, including my own, the more tired I am and less rested mentally and physically, the slower work gets done, to the point where it's not feasible to do it sometimes as it's very much thinking based and tiredness can hamper that greatly. Therefore more rest allows me to actually overall be more productive over the course of a working week.