That was my experience as well when I tried it 10 years ago. I started with Uberman, but had to shift to a schedule with 4 hours of core sleep towards the end

That was my experience as well when I tried it 10 years ago. I started with Uberman, but had to shift to a schedule with 4 hours of core sleep towards the end of the second week, as I was getting severely sleep deprived and unable to function. I'd gone close to four days without sleep a few years prior, and the feeling was pretty much the same. My interest was sparked as I've always had trouble with my circadian cycle, and my free running sleep eventually evolved to a polyphasic rythm to keep up with work, study and social life. I eventually had to give that up, as I ended up having extremely vivid dreams that were hard to discern from reality. Getting a good nights sleep and possibly an afternoon nap really is the key.