Back then I generally only slept 4-6 hours per night and when I wasn't trying to set a schedule, my sleep schedule would shift at least an hour a day. I was also drinking quite often and sometimes would stay awake for 24 - 36 hours at a time (coffee and booze only). All that is to say my sleep schedule in my twenties was a roller-coaster.
I got lots of work done. I got lots of partying in. Sometimes (many times) I mixed the two.
I say all that to say that I had a lot of freedom to play with my sleep schedules, and I tried quite a few.
I was able to do the two hours of sleep with four hours awake schedule for 2-3 weeks at a time and it generally worked pretty well as long as I adhered strictly. Getting less sleep than that at a time didn't feel right at all. If I pushed either the awake or sleep time, my body would automatically try to escape the schedule and it would be hard for me to get it back. The hardest part was fitting things into 4-hour blocks. If it was a stretch with nothing but work - like during some nasty winter weather - then it was easy, but as it's been said elsewhere, that gets hard to do when you have a social life.
When my son was born, I decided to finally lock in my sleep schedule - for the first time since I was a teenager. I was sleeping from 4am until noon, which allowed me to care for my son at night while my wife slept. This was key to a happy marriage, by the way. We were both getting 6-8 hours of uninterruped sleep a night with a newborn!
Once he was sleeping through the night (4-months-old, sleeping 12 hours a night!), I shifted to a "normal" schedule of 11pm-7am. While switching, I became so completely stuck into a 2x4-hour sleep schedule that it took about 3 weeks for me to get past and shift my sleep. It seems my body got so used to 7-8 full hour stretches, that it fought me from changing it. I suddenly understood why people have such a hard time with sleep! In my 40+ years of life, that was the hardest sleep schedule change I've ever made!
These days I do everything I can to get 7-8 hours and because of my boy I try to make sure that sleep is at night. I miss my revolving sleep schedule. I miss working at night. But my memory is far better. My caffeine intake is almost non-existent except for pleasure. My relationship with my wife is _probably_ better. And I get to spend time with my son every day, so it's a huge win.
As a side note, this sort of sleep schedule can be hell with a significant other, as I've dealt with plenty of times over the years. I'm very fortunate that my wife doesn't mind me shifting my schedule however I want, whenever I want, as long as I make time for her in my "day".