I used to have this problem in one of my favorite classes as a high school student at around the same time of day you are describing. It can be a combination of things--when you're eating lunch, working out, and Circadian rhythm are all factors to watch for.
As for acknowledging this to your professor: if she or he has seen you fall asleep, then I would say yes. As a professor myself, I would be empathetic with the student. I would also feel better that it isn't about my lecture or course plan, but rather, that it's something physiological in a given day.
Here are some tips that help me when I notice I'm getting that afternoon slump:
Oranges are the fastest way to get a boost of energy (old sprinter's trick from my track days...), so you might even bring slices to class or eat them just before if you know you're getting a bit tired.
Caffeine can help, but be sure to pay attention to when you are drinking it. Some tea/iced tea with lunch could be good.
Change up the time you're eating lunch at and/or make sure what you're eating for lunch is plant and protein forward.
Change some of your exercise routines. For me, a regular yoga practice is the best regulator of my sleep. I do also like evening spin or a long walk when the weather permits. As a yoga instructor, I find that for myself and my yoga students, this is the best thing for less lethargy during the day. You could even do morning practices instead of night practices, but I think you'll find it really helps your body with those Circadian things.