Day 5 here - I am not an athlete by any stretch (run maybe 3 times a week 5-8K) and my work isn't strenuous. I had to tail back the running the first 4 days, as I had energy but was a bit 'brain slow' and dizzy.
I am ramping more strenuous exercise back up now. Not sure if this helps, but one thing I have found about the Master-Cleanse - is it is different for everyone.