bright and early, long ago, early in life
夙 centers on doing something early: rising bright and early, acting long ago, or starting early in life. It conveys a sense of being ahead of the usual time.
夙 combines 歹 (remnant, bare bones) with 几 (table, enclosure). The exact historical origin is uncertain, but it likely depicted something done in the early morning or at dawn, perhaps related to preparing or working while it is still dark.
Imagine a bare-bones remnant (歹) of the night still lingering inside the enclosure (几) of dawn. You rise bright and early, before the darkness fully fades, getting a head start on the day.
For シュク, picture a shoe (シュ) that you put on quickly (ク) in the early morning. The shoe-cue (シュク) reminds you to start bright and early.