I think the biggest problem/excuse/justification is that life makes it difficult to go to bed before XX o’clock (8:00 PM for me). I’ve never done this bifurcated sleep method, but I am happiest and most productive if I go to bed around 8 PM and rise around 3:30/4:00 AM to workout and get on with my day. But then I have work problems that require a later evening, or a family member that wants to chat, or a house repair I just have to tend to, of a close friend with kids that I only talk to every few weeks who finally has a free moment… and the whole schedule gets screwed up, all the time. The going to bed early, regularly, is the difficult part I think.

Like others have pointed out, It's really difficult to operate on a different schedule than the rest of society. Eventually family, friends, and other activities pushed me to give up my evening "first sleep"