Why do morning people have shorter biological clocks?
Circadian rhythms are entrained by light via the suprachiasmatic nucleus, a part of the brain that receives signals from special retinal ganglion cells that are directly sensitive to (mainly blue) light.

However, light is not strictly necessary: the internal circadian clock is the result of shifting gene expression that proceeds without outside stimulus. Light is only used to entrain (or "reset") the clock each day.

When people talk about the duration of an individual's circadian rhythm, they are referring to what happens in the absence of a light cycle (that is, without entrainment).

https://en.wikipedia.org/wiki/Circadian_rhythm
https://en.wikipedia.org/wiki/Suprachiasmatic_nucleus
https://en.wikipedia.org/wiki/Circadian_rhythm_sleep_disorder