I read this on The Huffington Post--excerpt from an article by Scott B. Kaufman and it rings true, for me at least.
Creative people have a great deal of physical energy, but they're also often quiet and at rest. They work long hours, with great concentration, while projecting an aura of freshness and enthusiasm...This does not mean that creative people are hyperactive, always "on." In fact, they rest often and sleep a lot. The important thing is that they control their energy; it's not ruled by the calendar, the dock, an external schedule. When necessary, they can focus it like a laser beam; when not, creative types immediately recharge their batteries. They consider the rhythm of activity followed by idleness or reflection very important for the success of their work.
Creative people tend to be both extroverted and introverted. We're usually one or the other, either preferring to be in the thick of crowds or sitting on the sidelines and observing the passing show. In fact, in psychological research, extroversion and introversion are considered the most stable personality traits that differentiate people from each other and that can be reliability measured. Creative individuals, on the other hand, seem to exhibit both traits simultaneously.
Creative people's openness and sensitivity often exposes them to suffering and pain, yet also to a great deal of enjoyment... Being alone at the forefront of a discipline also leaves you exposed and vulnerable.