I think that manic-depression is extremely common among startup entrepreneurs. The emotional roller-coaster one goes through when trying to create a company exacerbates any bipolar tendencies that might already be there. The great news is that when one is in the state of hypomania they are often at your most creative and productive. Read this description from a well-known, UK-based charity in the UK:
"When I'm in a manic phase, I feel as though I am capable of anything and everything. This can be an amazing feeling, but I sometimes get frustrated and angry with people. Ideas flow constantly and quickly, as if my brain is on fast-forward. Everything happening in the world has significance in my life. But when I'm depressed, it's as if I'm completely crushed and living in slow motion. I feel capable of nothing.