I say, and with God's help: Wudu (ablution) is invalidated by sleeping in a lying position, or leaning, or resting against something, such that if that thing were removed, the person would fall: sleeping in a lying position means to sleep with one's sides on the ground, and sleeping while leaning means with one of the thighs. However, if a person sleeps cross-legged, or in a squatting position, or while praying standing, bowing, sitting, or prostrating, their wudu is not invalidated; because in these postures, relaxation does not reach its limit, unlike the earlier cases. Drowsiness is of two types: heavy, where one does not hear what is said around them, and this is an event in the case of lying down. And light, where one hears what is said around them, and this is not an event in the case of lying down. Narrated from Ibn Abbas: "He saw the Prophet (peace be upon him) sleeping while prostrating until he snored or breathed heavily, then he stood up to pray. I said, 'O Messenger of God, you have slept.' He said: 'Wudu is only required for one who sleeps lying down, for when one lies down, their joints become relaxed.'" This is found in Sunan al-Tirmidhi 1: 111, Sunan Abi Dawood 1: 52, Sunan al-Daraqutni 1: 159, Musnad Abi Ya'la 4: 477, Musnad Abd ibn Hamid 1: 220, and Al-Mu'jam al-Kabir 12: 157. Ibn al-Mulaqqin said in Al-Khalasah 1: 53: "It is weak." In Majma' al-Zawa'id: "Its narrators are trustworthy." See: I'la al-Sunan 1: 129. And from Abu Huraira: "There is no wudu required for the one who is sleeping while leaning, nor for the one who is standing and sleeping, nor for the one who is prostrating and sleeping, until they lie down; when they lie down, they should perform wudu." This is in Sunan al-Bayhaqi al-Kabir 1: 122. Ibn Hajar said in Al-Talkhis 1: 120: "Its chain is good, and it is a stopped narration." And from Amr al-Shu'ayb from his father from his grandfather, he said: "There is no wudu required for one who sleeps sitting until they lay their side on the ground." This is in Al-Kamil 6: 467. Al-Qari said in Fath Bab al-Niqayah 1: 66: "These hadiths, although individually they are not free from weakness, when they support each other, they do not drop below the level of good, and there is no explicit contradiction to them, so it is permissible to act upon them." And from Ali ibn Abi Talib, the Prophet (peace be upon him) said: "The seal of the eyes is sleep; whoever sleeps should perform wudu." This is in Sunan Abi Dawood 1: 52, and it was graded as good by Al-Mundhiri, Ibn al-Salah, and Al-Nawawi, as mentioned in Nasb al-Rayah 1: 45, I'la al-Sunan 1: 130. And from Ibn Umar: "Whoever sleeps lying down must perform wudu, and whoever sleeps sitting does not have to perform wudu." This is in Musnad al-Shafi'i 1: 228. See: Umdat al-Ri'ayah 1: 76, and Tabyin al-Haqaiq 1: 10, and Majma' al-Anhar 1: 20, and Al-Ikhtiyar 1: 15, and God knows best.