Mishnah Bava Kamma 2:6: “A human being is always mu‘ad [strictly liable; see Exodus 21:36], whether he behaves accidentally, deliberately, awake, or asleep….”
Bartenura: “If someone is sleeping and another comes and sleeps beside him, and the second one damages the first, he is liable. And if the first damages the second, he is exempt. And if they fall asleep together, whoever damages his fellow is liable, because they are both mu‘ad with respect to each other.”
Personally, I think this another excellent reason to go to bed early.