Before we move to the next verse, let us clarify the first sentence of verse 12: سَخَّرَ‌ لَكُمُ اللَّيْلَ وَالنَّهَارَ‌ (And He has subjugated for you the day and the night). Here, 'subjugating the day and the night' means that Allah made them subservient to His command in order that they can serve man, pursuant to which the night brings comfort to him and the day smooths out the avenues of his work. 'Subjugating them' never means that the day and the night start alternating under human command.