Within the space of a single verse, the Torah twice refers to the night of the Seder as leil shimurim, a night that is guarded: It is a night that is guarded by G‑d to take them (Israel) out of Egypt, this night remains to G‑d a night that is guarded throughout the generations (Exodus, 12:42).

Our Sages offered a number of explanations of this phrase.

A night that is guarded: a night of anticipation and waiting, for G‑d guarded and anticipated this night when He would fulfill His G‑d guarded promise to take them out of the land of Egypt (Rashi).

A night that is guarded: a night that is specially set aside for a two-fold redemption: for G‑d and for His nation. [This explanation is based on the use of the plural shimurim.] We see that throughout the period of bondage, it is as if the Divine Presence were also enslaved in Egypt.

Moreover, we find that whenever Israel went into exile, the Shechinah went with them in their exile (Yalkut Shimoni, Exodus, 210).

A night that is guarded: a night that is reserved for the future redemption. Why is the word shimurim repeated in this verse? Because on this night, in other times and places, G‑d did great things for the righteous, just as He had done for Israel in Egypt.

On this night He saved Chizkiyahu from Sennacherib and his armies; on this night He saved Chananyah, Misha'el, and Azaryah; on this night He saved Daniel from the lion's den; and on this night Elijah and Mashiach are made great (Shemot Rabbah 18). This is why the verse ends with the words: It is a night that is guarded for all Israel for all their generations.

A night that is guarded: a night on which there is protection from harmful elements. For this reason we do not recite the entire Shema and the other prayers asking for G‑d's protection that are usually said before going to sleep. We read only the first paragraph of Shema because on this night we enjoy special protection from G‑d (Shulchan Aruch, Orach Chayim 481).

The Talmud (Pesachim 109b) notes that though we are enjoined to drink four cups of wine at the Seder, and this has a potentially deleterious effect, we may do so because this is a night that is guarded.

Ma'aseh Roke'ach notes that he heard of a great Sage who would never lock the doors of his house on this night.

He adds that it has become customary to leave the doors open so that we may go out to greet Elijah without delay, for it is written that Israel is destined to be redeemed on the night of Passover. It is a night that has been guarded and reserved for redemption, ever since Creation.

Magen Avraham, quoting Maharil, writes that while one should not bolt the doors, he may close them since a person should not rely on a miracle for protection.

A night that is guarded: Ibn Ezra writes that this means a night of guarding, of wakefulness, for it is customary to refrain from sleeping so that we might occupy ourselves with praises of G‑d and relate His mighty deeds when He brought us out of Egypt.

The Haggadah mentions the Sages of Bnei Brak who remained awake until it was time for the recital of the morning Shema.