Peace jkhan.
It is good to see you going through the whole Qoran and using your intellect ,pondering its verses.
This is commendable and the way GOD wants us to do it . Yes each individual is a special receiver of GOD s message to search/analyse it.
Coming back to your query, I can only give my take as follows:
GOD has made the day and night alternating and changing in length as well throughout the seasons/year.
GOD said:
25:62
He is the One who designed the night and the day to alternate: a sufficient proof for those who wish to take heed, or to be appreciative.
وَهُوَ الَّذى جَعَلَ الَّيلَ وَالنَّهارَ خِلفَةً لِمَن أَرادَ أَن يَذَّكَّرَ أَو أَرادَ شُكورًا
Before we deal with 51:17-18, I have pondered the above verse and I have found so much detail in there:
1- Day or night can go from 5/6hours of length to 18/19 hours . In extreme cases-north pole/south pole - day/night can stay weeks/months longer.
2- GOD has designed this and gave us instructions ,deen that requires discipline to deal with the flexibility of day and night.
3-For a believer ,business /work is conducted- in most cases-in the day. Night is for rest/sleep meditation ,reflection ,ideal for its stillness and quietness
In context, one need to ask this: How long is a little of the night? How much sleep does an adult require as a minimum?
In my opinion GOD is asking us to spilt the night appropriate to its length ,and still take the minimum you need for sleeping.. Of course GOD is not saying do not sleep or sleep for so long. So I take a "little" to mean sleep some/minimum time, the rest is for meditation/reflection/remembering GOD during the ideal conditions of the nights
More in longer nights,less in short nights as sleep is also a necessity.:
He is the One who designed the night to be a cover, and for you to sleep and rest. And He made the day a resurrection.
وَهُوَ الَّذى جَعَلَ لَكُمُ الَّيلَ لِباسًا وَالنَّومَ سُباتًا وَجَعَلَ النَّهارَ نُشورًا
GOD also said to" remember GOD frequently all the time" it does not mean do not work ,sleep or use your time for leisure and to spend with friends,but "our god is whatever is in our mind 24/7".
So I understand by "little" , a minimum of hours- 4 to 6- of sleep. This is also a necessity that GOD designed for us.
And of course, these days there are many who work nights and sleep by day. A reverse for them of day and night is also fine.
The more we know GOD, the more we remember GOD and the more we study,ponder and follow His instructions with a balance that allows us to live our worldly life as well.
Best of luck to you brother.
GOD bless you.
Peace.