Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the verse of Surah al-Muzzammil, 'Stand (in prayer) through the night, except a little, half of it' [Surah al-Muzzammil: 2-3], was abrogated by this verse: 'He knows that you will never be able to keep it up, so He has turned to you in mercy; so recite as much of the Qur'an as is easy for you' [Surah al-Muzzammil: 20]. This was their prayer in the early part of the night. He says: This is more suitable for you to be able to keep count of what Allah has made obligatory upon you of the night prayer, (for) when a person goes to sleep, he does not know when he will wake up. And the saying of Allah, 'more upright in speech' [Surah al-Muzzammil: 6]: this is more fitting for one to understand the Qur'an. And the saying of Allah, 'Indeed, for you in the day is prolonged occupation' [Surah al-Muzzammil: 7]: by 'sabhan tawilan' is meant ample free time.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4639