Check out this post on Brain Based Business called, Brains Under Pressure. Here is a snippet:
A fine colleague of mine just resigned from several boards ... and cut back on work hours. His doctor warned that stress has spiked his blood pressure to life threatening levels. Have you seen it happen?
We can do only so many things well and with full engagement. If we spread ourselves too thin, we never get into anything to the degree where we can reach a flow state.