I say, and with God's success: If he wakes up before his fall, or at the time of his fall, or falls asleep and wakes up immediately, his ablution is not invalidated. However, if he remains asleep after the fall and then wakes up, his ablution is invalidated; due to the presence of sleep while lying down, according to the more correct opinion, as stated in Al-Zahiriyya. Refer to Majma' Al-Anhar 1: 20, and this is the apparent view as mentioned in Al-Tahbiyyin 1: 10, and God knows best.