Al Isra (The Night Journey)
Surah 17 · Verse 12
And We have made the night and the day two signs, then We darken the sign of the night and make the sign of the day visible that you may seek the grace of your Fosterer and that you may know the numbering of years and (keep) the account (of your works). And We have explained everything in detail.
Days and nights are for earning our livelihood and taking rest, respectively. They also provide a basis of keeping an account of time in days, months, years etc.
