What is the difference between the qiyam al-layl mentioned at the beginning and end of Surat al-Muzzammil?  The opening verses of the surah states, “O you who wraps himself [in clothing], arise [to pray] the night, except for a little - half of it - or subtract from it a little” (Quran, 73: 1-3), while in the last verse Allah Almighty says, “Indeed, your Lord knows, [O Muhammad], that you stand [in prayer] almost two thirds of the night or half of it or a third of it, and [so do] a group of those with you” (Quran, 73: 20).

The qiyam al-layl (prayers performed at night after the obligatory nighttime prayer and before the dawn prayer) mentioned at the beginning of the surah refers to night worship that was made obligatory upon both the Messenger of Allah (peace and blessings be upon him) and his Ummah. In the closing verse, however, Allah Almighty eases the obligation for the Ummah, making qiyam al-layl voluntary, rather than obligatory.

And Allah Almighty knows best.