I do agree that maintaining a consistent 5 hours a night is, to say the least, a difficult task. There will most likely be lapses from time to time; that's to be expected. This being said, if you work at a high level of efficiency, even with those discrepancies to account for, the turnover should be significant enough.
My planning does not only cover academic guidelines I want to follow, but rather I try to include any aspect that may have an effect on my output. I have certain measures to manage physical and mental health, which take into consideration as many realistic obstacles as possible, including many of which people have raised in this thread. And of course, the most important aspect of any plan is flexibility. I am prepared to and even expect to deal with various changes that have not yet presented itself.
Everybody is different, with their own limits, tendencies, habits and so on, but what remains a common element for all in their bid for success is the fundamental logic of compensating for these variable shortcomings. Understanding that you have failings, that these failings can be overcome and thus working to address them is crucial to achieving any particular objective, however minor or grand. This, I believe, is key to unveiling the avenue towards your aim.