It may be that the course taught by the teacher is too difficult. After listening to it, I can't understand it, and my attention is distracted, so I feel sleepy.

It is also possible that the weather is hot, the brain lacks oxygen, sweats more, and the excretion of trace elements such as potassium in the body is more than usual, which makes the metabolism in the body disordered and easy to get sleepy.

You can try to preview the course first, read what the teacher is going to say in advance, and get to know it, so that you can keep up with the teacher's ideas in class and concentrate, so that you are not easy to get sleepy. If you are tired because of the weather, you can buy some refreshing herbal tea and have a drink between classes.