The surah can be divided into the following three sections.
Section I (1-14)
The Prophet (sws) is directed to stand regularly to pray by night and to recite in slow measured tones the Quran in this tahajjud prayer. This shall soothe and strengthen his mind and soul and enable him to bear the heavy burden of responsibilities, which shall be entrusted to him during the course of his mission of Prophethood. In the day time also, there is an opportunity for him to glorify the praises of his Lord, so he should keep in remembrance the name of his Lord and devote himself to Him wholeheartedly; He is the Lord of the east and the west and so he should entrust all his matters to Him. With patience should he ignore the nonsensical acts of his enemies and leave their matter to Allah, who is enough to take account from them.
Section II (15-19)
The Quraish are warned that just as the Almighty had sent the Prophet Moses (sws) to the pharoah to bear witness to the truth before him, He has also sent towards them a Prophet. They should keep in mind that the fate of the pharoah and his nation and if they also adopt the attitude the pharoah had adopted, their fate will be no different. They should remember the dreadful Day when the children will become grey-haired---the Day with whose burden the sky is cleaving asunder. The Quran is an admonition which is meant to remind people of this great event. Therefore, he, who wants his own well being, should remain on the right path, otherwise he should get ready to face dire consequences.
Section III (20)
The last verse of this surah was revealed in Medinah. In it, the directives of the tahajjud (late night prayers) mentioned at the beginning of the surah have been modified---being reduced in their extent. However, with this reduction certain other alternatives have been suggested which would be helpful in achieving the real purpose in the new circumstances which have arisen.
(Translated from Islahi's "Tadabbur-i-Quran") -- Shehzad Saleem