I slept 4 hours for about a month. It was a great thing to do--once. Now I know how little I can sleep and still function.

I think one thing I've learned though more than anything is that startups take a long time and its better to work from 8-10 7 days a week for 2 years than to work 6-12 everyday for a few months.

So I average about 6 hours a sleep a night now, and make sure to exercise everyday.