I tried biphasic for 1 week. It worked, I slept 3 hours from 2am to 5am and had a 1,5hrs nap at 6pm, but all the time I was feeling tired at 2-4pm. Maybe I didn't adapt well, but I work more productive when I've just awoke, and this schedule permits the whole 90min cycle, so I'll be refreshed all the rest of time. I mean, I sleep 90min every 4,5 hours, and I think that's a good ratio; my brain will better process all the data received throughout the day. With biphasic & monophasic, the brain receives more information than the sleep cycle (3 and 1,5 hrs) can handle, so it's harder or impossible to adapt ('cause you always feel tired, at least the first week).