I tried this a couple of years back. It resulted in some VERY strange waking behaviour - not actual tiredness, just a constant background awareness that you're not functioning efficiently.

My guess is the lack of really deep 'delta' sleep gradually eats away at the body's capabilities, much as (for example) drinking a bottle of wine a day does - it falls short of nonfunctioning, but definitely reduces effectiveness, possibly by more than the 40% of time you're saving. If you try it, I recommend cutting down on the whole leaving-houses-by-topfloor-windows thing - I hear it's prone to disaster...

Chris - interesting! How long did you do it for? What was your protocol?

Steve P hypothesises that the capacity to adapt to a substantially reduced sleep schedule might be affected by diet. Carnivores have to sleep a whole lot more due to the digestive burden; herbivores get away with much less sleep. He's vegan. Caffeine also seems to be a deal breaker in even small quantities: even a cup of green tea which has much less caffeine than coffee prevented REM during naps.

I've been caffeine-free in drinks since hospital this year and for this polysleep experiment will avoid any source of brain chemistry-altering foods like chocolate etc. Which is a nightmare at this time of year.

I've felt less than 100% over the last few days but it's been getting better; I blew through the two mid-night naps this morning without a problem and feel totally OK right now. Reading Steve P's logs things took a few weeks to get settled.

Just so that you're aware, there have been polyphasers who've gone for longer than a month.
One would be Puredoxyk (add a .com for her website) who did Uberman for 6 months during college, and is now on the Everyman variation (3 naps per day with a short 'core' sleep at night). Steve Pavlina also did this for 6 months, but quit due to it causing social inconveniences (again, add a .com for his site). I'm also a member of the Google Group for polyphasers and some of those have been doing it for several months, or have done in the past. Most people seem to choose Everyman for convenience, but a few have the willpower and circumstances which allow them to live 22 hour 'uberman' days.