The surah lightened its own command
This is the anchor text for the whole topic, and most people have only ever heard the first half of it.
Surat al-Muzzammil opens by commanding the standing of the night, and it is demanding: stand the night except a little, half of it, or a little less, or add to it. Then, at the end of the same surah, comes 73:20. He knows that you are unable to do it, so He has turned to you in forgiveness, so recite what is easy of the Qur'an, and among the reasons named are the ill, those travelling, and those fighting.
The Qur'an instituted a hard night regimen and then lightened it by name, and illness is one of the grounds it gives. If you believe the night vigil is a fixed standard you are failing, you have stopped reading before the end of the surah.
Qur’an and Sunnah
يَٰٓأَيُّهَا ٱلۡمُزَّمِّلُ
You [Prophet], enfolded in your cloak
Qur’an 73:1
قُمِ ٱلَّيۡلَ إِلَّا قَلِيلࣰ ا
Stay up throughout the night, all but a small part of it
Qur’an 73:2
۞إِنَّ رَبَّكَ يَعۡلَمُ أَنَّكَ تَقُومُ أَدۡنَىٰ مِن ثُلُثَيِ ٱلَّيۡلِ وَنِصۡفَهُۥ وَثُلُثَهُۥ وَطَآئِفَةࣱ مِّنَ ٱلَّذِينَ مَعَكَۚ وَٱللَّهُ يُقَدِّرُ ٱلَّيۡلَ وَٱلنَّهَارَۚ عَلِمَ أَن لَّن تُحۡصُوهُ فَتَابَ عَلَيۡكُمۡۖ فَٱقۡرَءُواْ مَا تَيَسَّرَ مِنَ ٱلۡقُرۡءَانِۚ عَلِمَ أَن سَيَكُونُ مِنكُم مَّرۡضَىٰ وَءَاخَرُونَ يَضۡرِبُونَ فِي ٱلۡأَرۡضِ يَبۡتَغُونَ مِن فَضۡلِ ٱللَّهِ وَءَاخَرُونَ يُقَٰتِلُونَ فِي سَبِيلِ ٱللَّهِۖ فَٱقۡرَءُواْ مَا تَيَسَّرَ مِنۡهُۚ وَأَقِيمُواْ ٱلصَّلَوٰةَ وَءَاتُواْ ٱلزَّكَوٰةَ وَأَقۡرِضُواْ ٱللَّهَ قَرۡضًا حَسَنࣰ اۚ وَمَا تُقَدِّمُواْ لِأَنفُسِكُم مِّنۡ خَيۡرࣲ تَجِدُوهُ عِندَ ٱللَّهِ هُوَ خَيۡرࣰ ا وَأَعۡظَمَ أَجۡرࣰ اۚ وَٱسۡتَغۡفِرُواْ ٱللَّهَۖ إِنَّ ٱللَّهَ غَفُورࣱ رَّحِيمُۢ
[Prophet], your Lord is well aware that you sometimes spend nearly two-thirds of the night at prayer––sometimes half, sometimes a third––as do some of your followers. God determines the division of night and day. He knows that you will not be able to keep a measure of it and He has relented towards all of you, so recite as much of the Quran as is easy for you. He knows that some of you will be sick, some of you travelling through the land seeking God’s bounty, some of you fighting in God’s way: recite as much as is easy for you, keep up the prayer, pay the prescribed alms, and make God a good loan. Whatever good you store up for yourselves will be improved and increased for you. Ask God for His forgiveness, He is most forgiving, most merciful
Qur’an 73:20
نِّصۡفَهُۥٓ أَوِ ٱنقُصۡ مِنۡهُ قَلِيلًا
half, or a little less
Qur’an 73:3
أَوۡ زِدۡ عَلَيۡهِ وَرَتِّلِ ٱلۡقُرۡءَانَ تَرۡتِيلًا
or a little more; recite the Quran slowly and distinctly
Qur’an 73:4
Translation: M.A.S. Abdel Haleem, The Qur'an, Oxford University Press, 2004.