I notice that all the students in my class seem very drowsy…

I notice that all the students in my class seem very drowsy 45 minutes into my class period. Knowing that it could not possibly be my lecture putting them to sleep, I consider other factors such as the fact that my class is at 5 p.m. and the sun is usually setting at that time of the day, which I think may make people drowsy. What type of evidence am I collecting in making this observation?