@SexismInManga People wake up at night all the time between sleep stages, we just fall asleep right after and don't remember it, it's called sleep arousal. Therefore I can believe that someone could kiss their loved one in their bed at night without waking up. The situation right here is less likely, because he kissed her only once before; this is not a habit. However he didn't sleep the previous night, worked at physically demandind job and attended lectures requiring brain power. He fell asleep at daytime, which is disruptive, and suffered from heat and nightmares because of broken AC. It is possible for him to wake up briefly and confuse dreams with reality.
Though I do agree that this is overrepresented in manga. As well as kissing / confessing to a «sleeping» person