Once in a while, you will find that you just cannot get going.
You try to work in different places but your body does not respond. You have coffee or eat nutritious food and start working, but you find the rejuvenation short-lived.
When this happens your body is telling you it is time to rest. When you go full speed for many days, your body will tell you it is time to refuel by resting a bit.
It boils down to balance. There is a time to work and a time to rest.