As the semester goes on, good resolutions fade. Among the students who still make the effort to come to class, I see some who desperately try to stay awake, but their eyes close in spite of themselves. (That happens to me too, especially if I listen to a talk right after lunch.) But what I teach is sometimes demanding. It requires an effort of the intellect.
When students do sports, at practice they have to put in the effort, day in and day out, even if sometimes it feels tiresome. But in class, even if they diligently put in the required effort to come to class, their brain might sometimes be reluctant to engage. They could just be too tired.
To awaken the brain from its slumber, the instructor can try to entertain. What if we had drills, like in sports? What if I asked questions and the students had to push some button to answer? Would that wake them up?
When I was in elementary school the teacher started the morning with operations. He or she would say: "3 plus 1" or "6 times 9", we would each write the answer on our small individual chalkboard, and raise the board high above our heads, for the teacher to see all the answers simultaneously. I always thought the purpose of that exercise was to memorize elementary arithmetic operations, but maybe there was something more to it: it woke up our brain, like drills waking up our body in sports. After a few minutes of such exercises, we, perhaps, felt more alert, more ready to listen to and learn new material.