Does sleeping invalidate ablution?
The measure of sleep to invalidate ablution is whether sit on the ground firmly or not. Accordingly, minor ablution is invalidated when a person lies on one side and sleep by staying in the position of prostration outside the prayer. However, sleeping in a state between sleep and wakefulness or in a position of sitting on the ground firmly and without one’s joints relaxed do not invalidate minor ablution (Mawsili, al-Ikhtiyar, I, 53). In this respect, for example, minor ablution of passengers, who travel by means of transportation by sitting up straight and sleep, are not invalidated.
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS