My hours tend to be unstable (long term), there will be months where I am getting in at 7am, and there are other months where I don't make it in before 10am. When I worked at a consulting company in the US, I was always in by at least 9am (usually 8:30am) because to do otherwise -- when you are on the client side - does not look very professional.
I believe the function of when you go to sleep depends on how long it takes you to wind down from work (if you are stressed, or have too many things on your mind -- then it is difficult to go to sleep -- and you require more hours post work to "relax" to the point of time that you are ready for sleep) -- which of course affects how early you can wake up.