Two comments on this point

1) I did preface that this is a loose framework that I run with and it's not set in stone - if I need more sleep on a particular day, I take it.

2) While all human bodies vary with their sleep needs, I find that I function best between 6 and 7 hours per night. While I can run on less at times, I do need to catch up. I find myself less focused when I start approaching the 8 hour mark.