The point is that there is an aspect of the circadian rhythm which can not be circumvented simply by going to bed earlier.

The abstract does a good job of summarizing why this matters: "During puberty, the adolescent circadian system naturally delays the onset of sleep to a later time. One reason for this is an apparent lengthening of the circadian period during the teenage years (1), which typically leads to a later onset of the biological night relative to the light-dark cycle"