Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.

 

Scholars are unanimous to the effect that if a fasting person wakes up during the day, even if it is for a single moment, his fasting is valid. If he does not wake up even for a second by spending the whole day sleeping but he has already intended fasting during night, the majority of scholars maintain that his fasting is still valid.

Allah Almighty knows best.