7.5 is the ideal for me. I do a lot of running and cycling though, which make me need more sleep. When I don't, I'm fine with 6 hours.

I also experimented with polyphasic sleep and it was too much of a pain as far as coordinating a schedule with the rest of the world.

What I did take out of it was that it was very important for me to sleep in multiples of 1.5 hours. So if I'm not feeling very tired, I'll go from 7.5 hours to 6 hours, but never to 6.5 or 8.

Best thing, I don't even need an alarm anymore. I just wake up naturally, which is great.