I get by with 4 hours per night. Used to sleep 7-8 hours, but managed to reduce my need for sleep after some biphasic/polyphasic experimenting.
To get by with only 4 hours, I do 3 hours core in the night and 2-3 20 min. naps during the day (around 6 am, 1 pm and 6 pm). My core is usually from 10 pm to 1 am.