This post features a comment (search the world "cultish" -- that's the one) in which someone reports to have tried polyphasic sleep and noticed a decline in memorization ability, as found through using Anki. Perhaps browse through their shared decks (install Anki and browse the list through the application or Ctrl+F via this method) and find something that is vaguely familiar or unfamiliar and try and learn it.
Honestly, you might be better off postponing until you get some good data before to make sure you have a non-affected control sample. Taking tests while conducting your experiment might make it difficult to gauge when any potential negative effects have worn off. At least you have a "default" opportunity now -- afterward, perhaps you should just pick an arbitrary time to recoup (like 3mos) and take the tests again (or pick an equally unfamiliar new topic to learn). Then compare your performance.
Lastly, what are your target criteria? Awareness? Learning abilities? Ability to follow conversation? Deep thought development? You might want to establish some clear goals beforehand -- what if you can memorize like a champ... but you develop twitches or feel like you're about to collapse all the time? Just saying -- think through your experiment before you do it... even if it means postponing your start. Better to do this on the front end.
My target criterion is having more time per day to do stuff, and that's it. A small decline in some cognitive abilities might be worth the significant increase in available time (if it's not permanent!).
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.