The nervous system is composed of parasympathetic and sympathetic systems and only one could work at any given time.
When people work too much, they use the sympathetic system almost exclusively and the body wears as it is not repaired as it should.
If you try to do too much, but your digestive system stops working at it should, your immune system stops working at it should, your sexual system stops working at it should, you are in a big problem in medium and long term.
In fact, people use artificial stimulants like coffee in order to mess and fool the natural mechanism of body repair. But they are fooling themselves because if you eliminate the signals but not the problem, you make the problem worse.
Science has studied elite sports in order to get better athletes and know we know that for every effort, you need a given resting time. Give no rest and the person can't compete in the elite, period.