Some have a custom to read Shir Ha-shirim (the Song of Songs) upon completing the Hagada, as it alludes to the love between God and Israel.
Though we have fulfilled our obligation to tell the story of the Exodus by reading the Hagada, there is a mitzva to continue embellishing the story and telling of the miracles and wonders that God did for our ancestors throughout the night of the 15th, until one is overcome with sleep. This mitzva includes studying the laws of Pesaḥ (SA 481:2; Gevurot Hashem ch. 2), but it does not include “pilpul” (talmudic casuistry) (Derashot Ḥatam Sofer p. 265).
It is best not to prolong the recitation of the portion of the Hagada that precedes the meal, because we want the children and all of the participants to remain alert until after the fourth cup. It stands to reason that when the Sages said that it is praiseworthy to prolong the telling of the story of the Exodus, it refers to the rest of the night, after the Seder is over.
If one fears that by staying up late he will be unable to pray Shaḥarit properly, he should nonetheless make and effort to continue relating the story of the Exodus until after midnight (Sidur Yaavetz; Kaf Ha-ḥayim 481:11).
Before going to sleep on Pesaḥ night, it is customary to recite Shema and the berakha of Ha-mapil, but not to read the other verses that are read on all other nights. These verses are recited as a protective remedy against harmful forces, but this night is safeguarded against such forces and opportune for redemption (Rema 481:2; and see Ben Ish Ḥai, Tzav 38).
May it be God’s will that just as we have merited to study the halakhot of the Seder, so may we merit to fulfill them.