It breaks you mentally bit by bit, and I personally experiencing this a few years ago on a startup. It was horrible and unexpected to find myself mentally unable to get my work done. Essentially it seems that the stress hormones released by constant physical & mental stress physically damages/alters the emotional centers of your brain. And over time the damage becomes more-or-less permanent. There is some research to support this, but it's a very new field (Some good research is coming out of the PTSD research being funded by the military.) I don't think it's far off to say that at least mentally, starting a company is a lot like going off to war. Less risk of physical injury perhaps, but similar risks for mental injury.
For healthy people, but particularly people genetically susceptible to stress-related disorders, startup work can trigger full-blown depression and bi-polar manic/depressive cycles. That's just part of the job, and you need to be prepared for it and take care of yourself.
IMO, successful founders are the ones that successfully cope with the inevitable manic/depressive cycles. Self-awareness of our limitations, and good medication are life-savers. Once you start testing out different medication options, prepare to spend up to a year finding a combination that works to help keep you stable through the most stressful and depressed times. Don't give up hope. Ever.