More on getting cranky if I don't write for a few days running: One of the odder books in my non-fiction library is Alice W. Flaherty's The Midnight Disease, a study of hypergraphia. That's the overwhelming desire to stop washing the dishes or to abandon the car or to leap out of bed and write. Write anything, at great length. Flaherty fell victim to the compulsion as part of post-partum depression, but after she recovered, she wrote this book on the connections between writing, mental health, and creativity.
Chapter two includes this sentence: "Modern researchers have found some experimental evidence that creative people have better access to primary-process thought. They fantasize more, have better memory of their dreams, are more easily hypnotized, and score higher on measures of mildly psychotic traits." It's not that madness and creativity go together; it's more that they are two routes that "access to primary-thought processes" can take. Some people take one and end up artists; some take the other route and end up crazy; some take both.
From my own observations, this rings true. The writers I know do fantasize a lot, can relate their dreams, and invest imagination with the same intensity as reality. I don't know about the "more easily hypnotized" part, never having seen any of them with a hypnosis expert. But they tell anecdotes with gusto and pursue idealized objectives with passion. And some of them do seem mildly psychotic, in that their perceptions don't always seem firmly Velcro-ed to reality.
I may or may not include myself in this group, depending on the day.