In 1693 Newton suffered another nervous breakdown. He began to send angry letters to his personal friends, including the writer Samuel Pepys and the philosopher John Locke, accusing them of things that were completely imaginary. In the case of Locke, Newton charged him with trying to entangle him with women. Both friends became alarmed and feared Newton was going insane, but shortly after he seemed to recover again.From This Web Bio.. . .
Years after Newton's death it was proposed that much of his erratic behavior might have been caused by mercury poisoning. He used a great deal of mercury in his experiments in alchemy and at the time, no one recognized the dangers. Recent studies of a hair sample from Newton showed he had forty times the level of Mercury considered normal.
It's the work, not the man.