 Although the ideal time for Tikkun Chatzot is the hour following midnight, "Tikkun Rachel" may be said until a half (seasonal) hour before `alot hashachar/dawn, and "Tikkun Leah" until dawn.The Magen Avraham method (also held by Rebbe Nachman of Breslov) is that midnight is six clock hours after nightfall (appearance of 3 medium stars).The method held by Mishnah Berurah is twelve hours after noon (halfway between dawn and dusk).