If you feel drowsy with 8 hours, try 7, not 9. If you're super sensitive, you can even try to fine-tune it, like 7.5 hours. But it sounds unlikely that you feel well sleeping 9, 10 hours regularly (barring some sort of condition). 

 maybe i'm just a master eeper

my record is 15 hours in a row (don't recommend)

i'll try out a regular time of going to bed and waking up without an alarm clock, to see what my body feels like