Well, I'm up to the full 21 now, and it seems to be going OK.
The fifth rite is still definitely the hardest, though so far I haven't had to stop. (Mind you, this is only my second day)
I'm starting to notice that my sleeping patterns are changing- I'm not sure if there's a link with these exercises. I slept for uncannily long periods over the weekend, though have started waking very early the last couple of days. All in a bit of a flux, really!