just a guess...
Just an educated guess, because I'm doing it too, but the 5 hour window is a part of the Fast5 IF plan, a freely downloadable e-book.
I have been doing it for the past four months with success. Since I work the 3-11 shift, I have allocated 11pm to 4am as my eating window. All my meals and snacks are VLC (very low carb)--very starter Atkinsy.
At 66, I am maintaining my reasonable weight and feeling very good. So, FOR ME, it works perfectly and I'll probably eat this way for the rest of my life. YMMV.