Is it Sunnah to seek forgiveness before dawn break?

Yes, Allāh has praised the believers (who do that). He said: "And those who pray and beg Allāh’s Pardon in the last hours of the night." [Āli ʿImrān 3:17] "And in the hours before dawn, they were (found) asking (Allāh) for forgiveness." [Adh-Dhāriyāt 51:18] And Istighfār is legislated always at all times.