OCCASION OF REVELATION
Ebne Abbas says: Revelation was not inspired to the Prophet (SA) for fifteen days. The idolaters said: Mohammad's Lord has forsaken and displeased him. If he is truthful and is commissioned by God, revelation should have been inspired to him regularly.
The above sura was revealed to reply them.
YOUR LORD HAS NOT FORSAKEN YOU!
VERSES NO.1-4
Here, it is sworn by light and darkness which could be compared with revelation of light of inspiration to the pure heart of Prophet (SA) and temporary cease of revelation which is sometimes necessary.
Then, it describes that God may deem it expedient to defer revelation sometimes. However, it does not denote that He has forsaken you or has become displeased with you. You are favored with God in the World and Hereafter.