Does sleeping invalidate ablution?

The criterion for sleep invalidating ablution is whether one is sitting firmly on the ground or not. Accordingly, sleeping while lying on one's side or in a prostration position outside of prayer invalidates ablution. However, dozing in a state between sleep and wakefulness, or sleeping while sitting firmly on the ground without one's joints relaxing, does not invalidate ablution. (Mevsılî, el-İhtiyâr, 1/10) Therefore, for example, passengers traveling on public transportation who fall asleep while sitting upright in their seats do not lose their ablution.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu