We are taught that the angel requested of Yaakov to let him free because it was after dawn. Often times the interaction between the spiritual and physical world is described as there being a metaphysical world above ours, then the mazalos underneath that, and the physical world at the bottom. The angels live in the metaphysical, the mazalos are considered somewhat physical and somewhat spiritual and they are they bridge between the two worlds, and our world houses the physical beings. In fact, each star/mazal has an angel above it and that angel influences the star which in turn influences the world. (Obviously Hashem is doing everything, but this is how the system of hashpo'o, how the heavenly forces come to earth, is described)
Due to this construct, often one sees similarities between the world of the mazalos and that of the angels. In this case it seems pretty clear. After dawn, the stars that are supposed to be out at night during that time of the year are present. After dawn they go away one by one as the powerful light of the sun drowns out their light. In the case with Yaakov, this angel was clearly supposed to be out at night during that time. However, as soon as dawn broke, it was clear that just like the star that represented him would be fading away, so too, he needed to be released to go back to the heavenly realm.