Does sleep invalidate ablution (wudu'), and what type and duration of sleep invalidate it, and does sleeping while sitting invalidate ablution?

Scholars have differed regarding the nullification of ablution by sleep. The preponderant opinion is that sleep is of three types: 1. Sleep of one lying down: This nullifies ablution, whether it is little or much, according to the four Imams. This is the preponderant opinion. 2. Sleep of one sitting: This does not nullify ablution if it is little. This is the madhhab of Ahmad, Malik, Al-Thawri, and Abu Hanifa. This is the preponderant opinion. 3. Sleep of one standing, prostrating, or bowing: Al-Shafi'i and one narration from Ahmad: It nullifies ablution. Abu Hanifa: The sleep of one who is in any of the postures of prayer does not nullify ablution, whether in prayer or not. The preponderant opinion: The ruling for the sleep of one standing is the same as for one sitting, and the ruling for one prostrating is the same as for one lying down.

There has been a difference regarding the definition of little and much sleep. The correct view is that there is no specific limit. So, whenever there is evidence indicating much sleep (such as the falling of one who was settled in his position), ablution is nullified. If there is doubt about its quantity, ablution is not nullified.