Question: If a person sleeps while sitting on a chair, will his wudhu become broken? What is the ruling if he sleeps away while sitting on a bench without a back rest?
Answer: if a person falls asleep while on a chair, his wudhu would not be broken. Similarly, if he sleeps away while sitting on a bench without a back rest his wudhu would not break, even though there might be some movement in his body due to his sleep. This law pertains to that position when a person’s buttocks are not removed from the surface on which he is sitting. If however, his buttocks were slightly removed while fast asleep, then in this case his wudhu would be broken.
(Shaami, Volume 1, Pg.141 / Tahtaawi, Pg.50 / AhsanulFataawa, Volume 2, Pg.23)
And Allah Knows Best.
Mufti Waseem Khan.