The 90 minute thing for monophasic sleep seems to be true. I was a little surprised how regular my sleep turned out to be: http://www.gwern.net/Zeo
At the start of the night, down into light sleep, then deep sleep, then up to light sleep, and finally some REM an hour or so later. Regular as clockwork, practically every night.
Incidentally, an interesting link (well, it would be if people would post more of their data) is the Zeo sub-forum dedicated to polyphasic sleeping: http://blog.myzeo.com/forum/polyphasic-sleep-experiment-discussion/
My friend and I are starting the Uberman sleep schedule (six 20-minute naps spread evenly throughout each day) tonight. Have other lesswrongians experimented with alternate sleep schedules? Are any of you qualified medical experts who can give input or advice? Success stories and failure stories would both be appreciated, and I'll keep you guys posted on our progress.