Imam Abu Hamed Al-Ghazali listed four apparent reasons and four inner reasons to help sustain one with night prayers:
The apparent reasons:
1- Not to over eat, because that will lead to over drink, which makes sleep overpowering and makes Night prayer heavy.
2- Not to tire oneself during the day in useless things.
3- Not to miss the aftrenoon nap (Qayloola), for it sustains your night prayer.
4- Not to engross in evil deeds, for it will diminish the desire for night prayer.
The inner reasons
1- Guarding the heart against hatred of other muslims, against innovations and against curiosity in the mortal Dunia.
2- Fear of God binding your heart, with shortness of hope.
3- To learn and know the virtues and benefits of the night prayer.
4- Love of God (strongest motivator), strength of belief in that every word in night prayer every letter he speaks is communication with Allah.
Additionally, anonymous, know that the best night prayer is indeed the last third of the night, before fajr comes, but that it is not the ONLY way of praying the night. If you have a structured day that is making it too hard to do so, then after you finish Isha and its Ratiba, start your night prayers and make your witr before going to sleep.
However if you do that, it is still good if you can wake up before fajr and catch a few moments of that golden hour to make suppllication and tasbeeh.
Allah al-mowaffeq