From what I've heard, Newton was, in the tradition of all great thinkers, a right proper lunatic, although some people believe it may be down to the large amounts of mercury found in his body after his death. According to
this page:
Newton continued on however, remaining the humble professor, but with increasing absent�mindedness. Friends laughed at him when visiting his home when they saw the sight of his front door. He had cut two holes in his front door, one for a cat and one for the cat's kitten. Often he would wander off from dinner guests after forgetting what day it was or after his thoughts had drifted away upon mathematics.
...
Finally, Newton spent the remainder of his life and career tackling mathematical problems that had long baffled other mathematicians. Often solving these problems in a matter of hours, he would send the answers anonymously to his peers, such as Bernoulli as a form of professional humor.
Apparently he also kept many of his discoveries secret for decades before publishing them (he only published Principia Mathematica because Halley badgered him into it).