Wow, with as often as I get asked about biphasic sleep, it's nice to finally have something to say about it. ;)
I've been using Everyman 6/1 (six hours' sleep and one nap) as a stop-gap on days where I miss two of my regular naps for a few years now, but I haven't felt like that gave me enough information to discuss it as a schedule — I know that Everyman 3 and Everyman 4.5 work for me, so if the biphasic E6 schedule was accumulating sleep-debt, I could be just fixing it by reverting to one of the other two — maybe even without realizing it; if I was tired the next day, I'd get an extra nap or two, sleep the corresponding 3 or 4.5 hours at night, and wham, fixed.
(REMINDER: I've been polyphasic for years, and wasn't able to switch schedules like this until I'd done it for over a year. Be consistent while you're adjusting to a new schedule!)
But I've been doing E6 a lot more often in the last few months — since I took this job, I suppose, which was three months ago. And while I still think I consider and prefer E3 or 4.5 as my regular schedule, I looked up the other day and realized that I've been on E6 for probably five days a week for the last several weeks, and I feel like I understand it better now.
My biphasic schedule looks like this: I sleep for six hours at night — which feels like a lot to me — and then I catch one 20-minute nap during the day, usually around 1-2pm. The schedule can shift, though; at least one day a week I seem to get too tired by about 9-10pm doing this to comfortably stay awake, so I sleep until 4am, and sometimes my next day's nap is my morning one, around 7:30. (When that happens, though, I'm likely to be tired enough to take my 7pm nap too, and then since I've had two naps, I only need 4.5 hours of sleep at night — you get it.)
As you probably know, most biphasic schedules involve a nap of longer than 20 minutes — often betwen 45 and 90m. While I can see needing a bigger chunk of sleep if it's to be your only one to break up 18 hours of being awake, I can't take longer naps. I wake up ridiculously automatically at around 18-19 minutes now, feeling refreshed and ready to roll. So even when I'm sleeping 6 hours at night, my one nap is still ~20 minutes long.
But overall, while I don't prefer biphasic sleep over one of the shorter Everyman schedules (or Uberman; let's not forget that I'd happily do Uberman if I could), and while I'm unsure that I could do just six hours' core plus one 20-minute nap every day and not become tired, I think I can say now that E6 is pretty viable as a sleep-schedule: Probably at least as viable as 8-hour monophasic sleep was for me, and quite likely moreso; plus it does take 1.5 hours a day away from the standard amount of overall sleep needed. There's already a ton of evidence that 4-7 hours at night plus one ~1-hour nap works (and I've never argued that it doesn't), but having done E6 as much as I have now, I guess I can add that this one works too, and that biphasic sleep in general is a pretty good thing.