Even the experts don't really know why, so you're just guessing, and taking a huge risk.
However, the evidence seems to be pointing to physiological damage directly caused by the lack of sleep, and that 7-8 hours is ideal for nearly everyone.
Again, ymmv and my account is anecdotal, but my father was (and is, aside from the diabetes) an extremely healthy person, both in habit and on paper (cholesterol numbers, etc.) That's why his diabetes was such a mystery.
It doesn't seem like you can out-health a lack of sleep any more than you can out-health radiation exposure. And like radiation, the effects may be cumulative.
Howany hours are you working per day? I'm doing 12+ so that's probably adding to the exhaustion factor a lot.
Might be interesting to try this without working insane hours haha.
Weekends I get anywhere from 8 to 12+ hours of sleep.
I get about 5 to 6 hours per night as well on average and I'm feeling fine. I workout regularly and don't smoke and rarely drink, physically fine as well and don't feel tired.
Your mileage may vary obviously but I think a lot of association of health risk with reduced sleep is probably attributable to unhealthy lifestyles overall (interestingly enough there's also higher associated mortality with excess sleeping)