The human body is built in such a way that it literally needs energy every single time, not just to do work, because even when you’re just sitting idle at a particular place, your organs, like the heart, livers, brain cells and other organs are working tirelessly to make sure you’re kept alive, and all these organs requires energy, so in a situation where your body lacks the energy, the organs gradually loses the capacity to function correctly and this can cause failure and thus death, which is why food is very essential to the body at least once in a day to refill the lost energy.
There is time for everything. There is a time the body needs to be functional and a time when the body needs rest. When we overwork the body without rest, it affects several parts of the body negatively, and it will get to a point where the body cannot take it any longer, but shuts down. The body will shut down either with illness or a deep sleep. If we do want to be effective and to live a healthy life, dont deny the body the necesssary rest it deserves. Its like an engine, like every engine it needs maintenance.