Verily, rising (Nashi'ah) at night is better for understanding and more suitable for speech (recitation).
`Umar, Ibn `Abbas and Ibn Zubayr, all said,
"The entire night is Nashi'ah."
Mujahid and others said the same.
It is said "Nasha'a" when a person stands at night to pray.
In one narration from Mujahid he said, "(It is) after `Isha' (prayer)."
This was also said by Abu Mijlaz, Qatadah, Salim, Abu Hazim and Muhammad bin Al-Munkadir.
The point is that Nashi'ah of the night refers to its hours and its times, every hour of it is called Nashi'ah, so it refers to the periods of time. The purpose of this is that standing at night (for prayer) is better for training the heart and the tongue, and more conducive to recitation. Thus, Allah says,
هِيَ أَشَدُّ وَطْءًا وَأَقْوَمُ قِيلً
is better for understanding and more suitable for speech (recitation).
meaning, more comprehensive for the matter of performing the recitation and better for understanding it than in the recitation of the day. This is because the daytime is the time for people to disperse and move about, to raise voices and be lively.
Al-Hafiz Abu Ya`la Al-Mawsili said,
"Ibrahim bin Sa`id Al-Jawhari told us that Abu Usamah told us that Al-A`mash informed us that Anas bin Malik recited this Ayah as;
هِيَ أَشَدُّ وَطْءًا وَأَقْوَمُ قِيلً
Verily, rising at night is better for understanding and more correct for the speech.
So a man said to him, `We recite it;
وَأَقْوَمُ قِيلًأ
more suitable for speech.
So Anas said to him,
`Most correct (Aswab), most suitable (Aqwam), the best for preparation (Ahya') and similar words are all the same (in meaning)."'
Thus, Allah continues saying,
إِنَّ لَكَ فِي اَلنَّهَارِ سَبْحًا طَوِيلً