Anki reps, mostly. I found that I could do proper review sessions for about 2-3 days and would hit an impenetrable wall. I couldn't learn a single new card and had total brain fog until I got 3 hours more sleep. That, however, would reset my adaptation.

The whole effect is a bit less pronounced on Everyman, but not much. It is however easier to add sleep when you already have a core.

I didn't notice any other major mental impairment after the initial sleep deprivation. I could (and did) play 16+ hours of BG2 and similar games each day and not break down. (I'm so grateful for the easiest difficulty setting now. One Uberman attempt took me 55 hours until I got any sleep at all and all my cognition went to hell. But I could still play HL2, although I got lost within an elevator.)

The other really bad thing about polyphasic sleep is impatience. Quote from my diary at the time (also notice the slight irritation):

Impatience is really getting annoying. Except for the short core I can't skip any time at all anymore. If something takes 6 hours, like a download for example, I will be awake (almost) all the time and have to wait. Every. Minute. Of. It. You see everything pass. Someone just went to bed and you want to talk about something? Prepare to sit there, for 8 hours or more, fully awake. Wrote some email and await an answer? You'll have memorized 500 digits of π before you get it. You can't skip anything, can't just hibernate a few hours. Once the sun went down, you'll sit in darkness, for 14 hours and more right now. If you are not president by day, superhero by night and mad scientist on the side, you'll be bored right out of your skull. Your puny hobbies are not enough for The Night That Never Ends, mortal!

Besides all that, it's cool and kinda works.