Hafiz Abu Samee'ah
Benefit:
....... It is established from many evidences that ablution (wudu) is nullified by sleep.
[ابو داود: 203، ترمذي: 96، ابن ماجه: 477، وغيره]
However, it is also established that the noble Companions (radi Allahu anhum) would fall asleep while waiting for the ‘Isha prayer, and then perform the prayer without renewing their ablution. [مسلم: 376، ابو داود: 200، ترمذي: 78]
On this basis, there are different opinions regarding this issue.......
According to us, the preferred opinion is that the sleep which is deep and in which a person’s consciousness and awareness are lost, nullifies ablution, whether it occurs while lying down or sitting. In reality, this is what is truly called sleep: that is, sleep is a heavy veil which, when it suddenly descends upon the heart, cuts it off from awareness of outward matters. As for that sleep in which a person’s consciousness remains, as commonly happens during a sermon or lesson, where the listener is both dozing and also perceives that he is listening, and in this state, if he is called, he immediately responds, and can even repeat some of the words he has just heard—then such sleep does not nullify ablution, whether it occurs while lying down or reclining. In fact, this is what is called nu‘as, i.e., drowsiness.
[فتاوي ابن باز مترجم: 49/1]
Source: Muwatta Imam Malik by Abu Samia Mahmood Tabassum, Page: 37