I say, and with God's success: Wudu (ablution) is invalidated by sleeping while lying down — that is, sleeping with one's sides on the ground — and by sleeping while leaning on one side — that is, on one of the thighs — and likewise, wudu is invalidated by sleeping while leaning or resting against something, such that if that thing were removed, the person would fall; because in these positions of sleep, relaxation reaches its peak. Ibn Umar (may Allah be pleased with him) said: "Whoever sleeps lying down must perform wudu, and whoever sleeps sitting does not need to perform wudu," in Musnad Al-Shafi'i 1: 228. Refer to: Umdat Al-Ri'ayah 1: 76, Tabyin Al-Haqaiq 1: 10, Majma' Al-Anhar 1: 20, and Al-Ikhtiyar 1: 15, and Allah knows best.