The morning sun never lasts a day

This would appear to be a variation of "Make hay while the sun shines". The sun might last for a day but the morning sun is different from the midday sun or the evening sun. For a start, the range of temperatures varies considerably. We might also be more enthusiastic and energetic in the morning. "Well begun is half done" so it pays to get off on the right foot.

"The early bird catches the worm" so it knows the advantage of an early start. There are many sayings that stress the importance of the beginning. The rising of the sun heralds the start of a new day, the cock crows and a new round of activity commences. Be part of it.