Not sure if there is a correct answer here, but here are some reasons:
1. They haven't slept well the night before, due to homework, projects, and even using devices (like our phone) for too long. Waking up early for school will result in sleeping students in class. Even the environment of the class can put them to sleep (ex. dim lights, quiet students, etc.)
2. They are bored. Someone who hates math or history will start to yawn as soon as the teacher begins explaining (ex. even if they were just so energetic in gym class the period before).
