Sleep nullifies ablution
Sayyidina Ibn Abbas (RA) narrated that he observed the Prophet (SAW) sleeping while he
was in prostration. He was snoring or taking long breaths. Then he stood up continued to
offer salah. He said, "0 Messenger of Allah!! You had gone to sleep.He replied "Ablution
is wajib for one who sleeps lying down because his joints are relaxed when he lies down."
[Abu Dawud 202]