I sometimes kiss my best friend in her sleep
How exactly does this happen? Why do you kiss your best friend in her sleep?
I always ask her first before we kiss.
A sleeping person cannot consent to a kiss. Being half asleep and kissing her is not acceptable behavior.
We are just good friends and that's all that it is. I'm just teasing her.
I understand that you might not mean anything serious by this.  However, you need to consider how she might react.  You're kissing her without consent and that could be viewed by some as a serious violation.