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.
The next push is to replicate the more positive result of environment enrichment, which has been shown in mice but not people. Basically, environment enrichment has been shown to help facilitate LTP (long-term potentiation — an extreme period of neurogenosis) and makes the hippocampus less prone to stress-induced atrophy.
I've personally been a few very high stress work environments, but I use this knowledge to help convince myself to relax productively.
Last year the [U.S.] Army's vice-chief of staff urged psychiatrists to change the term to "post-traumatic stress injury." Apparently, too many troops come home with PTSI but refuse to seek help. And as a result, some portion of them end up taking their own lives. The Army's hope is that renaming the phenomenon as an "injury" will reduce the fear of being stigmatized [0].
(Apparently some psychiatrists have rejected the idea of changing the name, because heaven forbid that they should alter their sacred naming convention for no better reason than that it might help some people.)
[0] http://www.pbs.org/newshour/rundown/2012/05/key-psychiatric-...
And so too is the daily stress level for most startup founders... There is a general high stress level involved in the day to day running of a business (particularly if you don't have prior experience doing it), but sometimes horrible things can happen in a business that make stress levels go through the roof.
I can't prove it, but I don't think it's too far off to say that the stress reaction founders experience at the really difficult points of their startup experience IS actually comparable to the stress reaction experienced by soldiers witnessing a horrible atrocity.
Being betrayed by a co-founder or investor. Having key employees quit. Lawsuits. Running out of cash.
Why is this accepted, considered common knowledge?
It doesn't have to be this way. Not at all. The vast majority of crazy stress related to "starting a company" is self-inflicted, through poor choices, poor preparation, and personality issues, like grandiosity.
Hey, that sounds like the reasons we go to war too!
Actually getting your job done and being a martyr are two different things, and you need to decide which you want. If you want the former, then when five p.m. comes around, stand up and walk away from the computer. Your work will still be waiting for you tomorrow either way, but this way you'll be fit to do it.