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/