Strange, I feel that by day 3, I'm fully switched over, and in fact I get a bit of a mental boost; I'm /definitely/ sharper on day 3+four, and then it tails off.

Mind you my typical fast starts on sunday after supper, and finished most fo the time on friday before supper. I do that so it doesn't impact too much the social life etc...

Overall I don't recommend short fast. I feel you need at least 3 days for it to be worth it, otherwise you get the pain (hunger on the first day) without a lot of the benefits.

But then again I only have my own experience on tap; most people I know think I'm bonkers to stop eating for most of a week ;-)