It frets, it excites, it consumes the body--as sand and grit, which occasion excessive friction, wear out the wheels of a machine.
Overwork and worry have both to be guarded against.

For over-brain-work is strain-work; and it is exhausting and destructive according as it is in excess of nature.

And the brain-worker may exhaust and overbalance his mind by excess, just as the athlete may overstrain his muscles and break his back by attempting feats beyond the strength of his physical system..