Congratulations on your will power! I hope this one goes well for you.
I have done a few fasts, and the second and subsequent ones were less daunting, probably because I knew what to expect. However, as you observe, the fist few days are the worst.
Physical weakness is normal. You do tend to sleep less when fasting too, so the yoga will probably help you sleep.
So the answer, I guess, is a combination of the two.
Hope you do well, my best advice (and I am sure others have already said it and you probably know this) is listen to your body :-)