In the ordinary condition of the body if you oblige the body to do too much work, it can do with the support of vital force. But as soon as the work is done, the vital force withdraws and then the body feels fatigue. If this is done too much and for too long a time, there may be a breakdown of health and strength under the overstrain. Rest is then needed for recovery.
If, however, the mind and the vital get the habit of opening to the Mother’s Force, they are then supported by the Force and may even be fully filled with it—the Force does the work and the body feels no strain or fatigue before or after. But even then, unless the body itself is open and can absorb and keep the Force, sufficient rest in between the work is absolutely necessary. Otherwise, although the body may go on for a very long time, yet in the end there can be a danger of a collapse.
The body can be sustained for a long time when there is the full influence and there is a single-minded faith and call in the mind and the vital; but if the mind or the vital is disturbed by other influences or opens itself to forces which are not the Mother’s then there will be a mixed condition and there will be sometimes strength, sometimes fatigue, exhaustion or illness or a mixture of the two at the same time.
Finally, if not only the mind and the vital, but the body also is open and can absorb the Force, it can do extraordinary things in the way of work without breaking down. Still even then rest is necessary. That is why we insist on those who have the impulse of work keeping a proper balance between rest and labour.
A complete freedom from fatigue is possible, but that comes only when there is a complete transformation of the law of the body by the full descent of a supramental Force into the earth-nature.
Ref : The Mother with Letters on The Mother