Musar su Esodo 10:21

This is the reason that we recite the paragraphs dealing with G–d having created the universe both in the mornings and in the evenings immediately prior to reciting the Kriyat Shema declaration. In those paragraphs we mention G–d as having created both night and day in order to demonstrate our conviction that they are part of the same whole and do not represent different domains presided over by different deities. This is why the Torah, when describing the process of creation, keeps saying ויהי ערב ויהי בוקר, "it was evening it was morning." Night is part of the unit called day and vice versa. Rashi explains on Exodus 10,21: וימש חשך, "that the darkness was tangible," that this was a phenomenon that intensified the normal darkness of the night. The night is darkest immediately before dawn. Before the sun sets in the evening, we observe the sun assuming a reddish colour in preparation for evening and night; the same process happens in reverse prior to sunrise. All this proves the basic unity of night and day. G–d is eternal, without beginning or end, without limitation of His powers, etc., as I have explained on other occasions. When G–d created the universe, He created "limits," as distinct from Himself who is limitless. In order to do this G–d imposed some restrictions, צמצום, upon Himself. The universe itself represents an act of G–d's self-restriction.