Through some studying of my sleeping habits (polyphasic sleeping for more than two months now etc.), I can say that the 90 minute rule breaks down around the 2 cycle mark - it is usually preferable to sleep 3.5 hours and not 3, particularly when you want to sustain a schedule that sticks to a main sleep that short.

This may not sound like a big deal, but for me, the difference has been night and day, where adding that half hour made the schedule sustainable in the first place.

http://trypolyphasic.com/forum/topic/992/ultimate-adaptation...