It depends on the position of sitting and sleeping. If a person sits leaning against his back and has a sound sleep, if the prop is removed one falls aside, then wudhu will be broken. But, if one slumbers while sitting, in such a state if he falls aside but he wakes up before falling or while falling, then wudhu will not be broken. But if he wakes up from his sleep after falling aside, then wudhu will be broken.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband