In another post, I wrote about a concept sometimes referred to as "muscle memory." You can read that post if you click this link; but essentially, muscle memory is a term that refers to your body and brain forming connections so that your body "remembers" how to do certain motions.
Essentially, according to Wikipedia, muscle memory is "a form of “procedural memory” that involves consolidating a specific motor task into memory through repetition, which has been used synonymously with “motor learning.”
In other words, muscle memory is how your body turns your practice sessions into "not wasted time" if you do it right, and "somewhat wasted time" if you practice wrong, because your body remembers the form and the detail of what you practice as much as the course motor movements. It really is worth a read.
One of the topics I addressed in that post was the importance of not practicing beyond your "fatigue limit," especially when working on tricky stuff - like trying to hit low notes when your lip is really tired. The reason is that in a case like that, your body is going to start to do all kinds of things (little things, but all working together against you) to compensate for the fatigue to try to get those notes out anyway, but if you can't control what you're doing, you're not going to control how the body remembered (wrongly) how to do it that last time you practiced when you were tired.