What is the hour of the night after which, even if one rak'ah is prayed, it is recorded as part of Qiyam al-Layl?

The time for Qiyam al-Layl (night vigil prayer) begins after the Isha prayer and continues until the appearance of the second dawn (Fajr al-Thani). Some scholars, like the Hanbalis, held that its time begins after the Maghrib prayer and ends with the appearance of the second dawn, based on the hadith of Anas regarding the verse: "Their sides forsake their beds" (Surah As-Sajdah, 32:16), where he said: "They used to perform nafl (supererogatory) prayers between Maghrib and Isha." Nafl prayers are valid with even a single rak'ah (unit of prayer), whether by night or day. However, it is preferable to perform them in sets of two rak'ahs, which is the relied-upon opinion among the Shafi'is and Hanbalis.