Tim Ferriss wrote a nice post called "Harnessing Entrepreneurial Manic-Depression: Making the Rollercoaster Work for You" on his personal blog.
I especially like the plot.
Tim explains each of these different stages: 1. Uninformed optimism, 2. Informed pessimism, 3. Crisis of Meaning, 4. Crash and Burn, and 5. Informed optimism.
Tim's discussion is aimed at business owners, but the plot applies to anyone doing creative work, including academics. Try and avoid the "crash and burn" stage!