I never tried any unusual sleep schedule, but I have worked third shift while trying to keep a social life.
It does get hard, even on a third shift schedule that millions of normal people deal with, to tell the days apart. When the day switches in the middle of your work shift, when it's light out all "night", even a normal third shift schedule makes the days run together. I did it for a year, and not a single day went by when it didn't feel like I took a short nap through the day then worked the rest of the time. Even when I got 8 hours of sleep.
I started drinking. I started smoking. I got short with my friends. I had to quit, because when there are no "days", life became meaningless to me. It was just one long night that lasted for a year. Things that happened ten minutes ago went into my memory as happening yesterday. When I quit my job and went back to sleeping normal, I stopped my self destructive habits literally overnight.
Millions of normal healthy people do it every day. But beware of forcing yourself into sleep schedules that you are not mentally prepared to handle. Know when to throw in the towel.