Almost everyone I know comes in to work sometime from 9:30 a.m. to 10:30 a.m. The Caltrains reaching SF from 9:05 to 9:30 are generally packed, so are the N and KT muni lines; so I am guessing it isn't only my social circle that starts their day early.
Of course I know a few people who start their day at 8:00 a.m. but it came to me as a complete surprise that the average is 8:17 a.m.