Why doesn't God call light light?

God called the light “day,” and the darkness he called “night.” Genesis 1.5 NIV

John Walton, Professor of Old Testament at Wheaton College, asks the following burning question about verse 5:
Why does God call the light day? Why doesn't he simply call it light? Hmmm.

Walton, like Lennox, points out that in this verse, light--as called day--is contrasted with night and thus represents a period of time. God called the period of light day. That makes sense.

Extending the logic back through verses 4 and 3 gives the reading, "God said let there be a period of light," and it was so. "We must inevitably consider day one," says Walton in chapter 5 of The Lost World of Genesis One, "as describing the creation of time." Oh! "The basis for time is the invariable alteration between periods of light and periods of darkness. This is a creative act, but it is creation in a functional sense, not a material one."

More explanation on this model is yet to come.