I am almost a week into biphasic sleep.
So far so good -- I am functioning off of 6 hours of sleep. But I'm hoping that my concentration and energy pick up. I've been maintaining the schedule fine (doing 1.5/4.5), but I noticed last night before my core sleep my brain just completely turned off. The adjustment stage seemed to have lasted only 3 or so nights. But perhaps by "adjustment" I simply mean "being able to wake up comfortably at the scheduled times." I don't think I've fully adjusted physiologically. It's not a huge deal, but my mental clarity feels slightly under par and I'd like to see that change within a week.
Another difficulty is that I seem to be conditioning myself to be a much lighter sleeper. I used to triumph over the fact that I could probably fall asleep in the middle of a busy intersection at noon. But just now I was just woken up from my 1.5 hour nap by soft chatter in an adjacent room. Since I didn't hit REM sleep, I'll have to re-attempt that nap soon (I'm guessing).
Of course, it's not a huge complaint, but I thought I'd contribute something more to the thread than just "hooray, biphasic sleep works!"