Have you ever wondered why people who are not as active are always the ones too "busy" to take on any other tasks?
From personal experience, the more active you are, the more you are able to take on.
I often use the analogy of your mind being like a sponge. There is a capacity of how much the sponge can absorb before it becomes ineffective; unless you do not squeeze out the water, that sponge essentially loses its purpose.
Similarly your brain, no matter how super human you are, has certain capacity it can handle before it too becomes ineffective. This is why it is important to keep ACTIVE, to allow you to detox from constant busy cycle of life and the everyday stress that accompanies it.
I notice the days I am inactive I always feel tired and exhausted, not able to think straight or the energy to complete, let alone take on additional tasks. However, the days that I am active and go to the gym - I feel energized and refreshed, my mind functioning well and ready to take on any challenge that I face. Mental fitness comes from Physical fitness.
A lot of times we are "thinking" more than we are "doing" making us feel busier than we actually are. A simple prioritization of what is more important makes all the difference. The more active you are, the more you are able to take on. Take care of your body it's the only place you have to live.