Quality of sleep is definitely a big factor, but genetics also govern how much sleep an individual needs. Some people just naturally need less sleep than others. Personally, I've found that if I allow myself to sleep and wake whenever I want, I eventually settle to a stable state of 8.5 hrs sleep/night. However, I typically get less than 8.5 hrs/night on weekdays and sleep in on weekends. After keeping track of my sleep, I found that on average, I was still sleeping 8.5 hrs/night every week, and I would sleep in just enough on weekends to hit my quota.