Here Allah begins explaining the evidence for His existence and His perfect power to create various things and pairs of opposites. Allah says:

Ibn ‘Abbas, Ibn Umar, Abu Al-'Aliyah, Abu Malik, Masruq, Mujahid, Said bin Jubayr, An-Nakhal, Ad-Dahhak, Al-Hasan, Qatadah, As-Suddi and others said, “This refers to the period from the beginning of the dawn until the sun rises.”

meaning, immobile, never changing. This is like the Ayat.

{Say : “Tell me! If Allah made the night continuous for you... ”} (28:71)

means, were it not for the sun rising, it would not be there, for a thing can only be known in contrast to its opposite. Qatadah and As-Suddi said, “The sun is a guide which follows the shade until the shade disappears.”

This refers to the shade.

{gradual} meaning slowly. As-Suddi said: “A gentle, concealed, withdrawal until there is no shade left on earth except under a roof or a tree, and the sun is shining on whatever is above it.”

{a gradual withdrawal.} Ayyub bin Musa said: “Little by little.”

It covers and conceals all things. This is like the Ayah:

{By the night as it envelops } (92:1).

{and the sleep a repose, } means, a halt to movement so that bodies may rest. For the faculties and limbs get tired from their constant movement during the day when one goes out to earn a living. When night comes, and it becomes quiet, they stop moving, and rest; so sleep provides a rejuvenation for both the body and the soul.

{and makes the day Nushur } meaning, people get up and go out to earn a living and attend to their business. This is like the Ayah:

{It is out of His mercy that He has made for you the night and the day that you may rest therein and that you may seek of His bounty...} (28:73)