Though it IS possible to get too much sleep, it's more likely that whatever amount you were trying to get was just interrupting a sleep cycle that left you feeling irritable and tired. Some people can get by on 6 hours and are okay; however, the vast majority who make do with 6 hours or fewer are setting themselves up for health problems in the future because they're living with a sleep debt they don't realize, and the fewer hours of sleep you get per night, the greater your chances of developing heart disease and diabetes. This is because sleeping less tends to make you produce more cortisol, which causes the body to pack away fat into your belly, around your major organs, which causes metabolic syndrome, which causes the diseases mentioned. So be productive, by all means, but be safe, too. Maybe you can try adjusting your sleep in quarter- or half-hour increments? That's what professionals usually suggest. Oddly enough, because it would put you in a different sleep cycle, 15 minutes may make all the difference in the world in the way you feel in the morning.

Good luck on your quest; it sounds like you're working it out :)