This alleviation wasn't granted without scientific evidence. I believe QinetiQ were involved in the research. Sleep trials were carried out etc etc.
As has been said above different people react in different way to early starts, getting out your bed at 4am is always going to be tough. Normally by the 2nd early I'm getting into a regular sleep pattern. Day 3 can be hard but day 4 and 5 are no problem for me. For others it appears different.
Under the old system of 6/3 by day 4 I was knackered as I'd transitioned onto lates right when my body was expecting to be in bed.
At the end of my 5 earlies I have two good days off - for others this is obviously different but the 3 days off at the end of the 6/3 system started at 1030pm when you finished on Day 6 and started "3" days later at 0515. Hardly quality time off.
Everybody is different but I find it ok. What is the perfect solution? 9-5?