I hardly get 8 hours of sleep a day. On a couple of odd days (mainly post diagnosis), I slept 14 hours because the medication made me drowsy.
My normal hours to sleep per night was 4 hours at University, 6 hours when I started working, 4 hours or less when I went travelling for 6 months. That's right, I went travelling for 6 months and I hardly slept but I was totally fine.
Doctors recommend 8 hours a day, but it is best to sleep what you require and have a quality sleep. I believe every night when I sleep it is quality sleep even if it is only for 4 hours.