I bite my tongue when i fall asleep sitting up, but not when i lay down. it doesn't really hurt, is it bad that this happens?

Of course; as stated: Tongue biting, especially repetitive, is clear pain-inducing, body-destructive, self attack. Two issues: (a) What is happening in your mind to drive this behavior while asleep in sitting position & (b) what & how much damage occurs each time along with how well (or not) the results of healing (never fully duplicating original anatomic structure/function before injury).