Being inconsistent negatively impacts your students capacity to learn, because they invest so much time , energy and effort trying to determine your mood for the day. You've been there. You know how it feels. Exhibiting mood swings is confusing and makes it extremely difficult for students to deal with you and maintain their need for balance and control.
Inconsistent behavior has an adverse effect on student behavior and performance.
For twenty plus years, I worked in the educational branch of the Texas prison system. Your students have the same questions and the same feeling because you are a pivotal person in their lives. Another lesson I learned while working with inmates is they prefer consistent behavior over pleasant behavior. What drove them crazy was to have a guard treat them like a human being one minute then like the scum of the earth the next. What your students crave is consistency in the way you treat them.