If people are feeling this way... it can in some cases be the start of burnout and fatigue.
I used to say things to people like "oh, I feel burned out a bit" and then I experience a deep, deep phase of burnout that was physically and mentally like anything I had experienced before - and it made me realize when I thought I was burned out, it wasn't really the case.
For me, the final descent started as that foggy headedness, and just general malaise - and then it got worse from there. It's such a hard feeling to describe, beyond just general exhaustion and mental fatigue and a feeling of "I just don't care..."
Make sure you are listening to your body and mind... take care of yourself and try to recognize it early.