Having an apple in the morning is a good idea. I'll have to give that a try.
I also know how you feel. Sometimes I get so exhausted in class too, but I'm scared to put my head down or anything because I sit right under the professor's nose (in all five of my classes). Why do I torture myself so?
I've been taking a B-12 stress tab with zinc. My doctor told me it would give me more energy, and it really seems to help. On the days that I forget to take it (like today) I really notice a difference.
Also, just be sure that you're getting the right amount of sleep at night. I can't seem to function on less than six hours, but my body really prefers 7.5. Too much more than eight hours- and I'll be sleeping all day.
I often take naps. I just can't help it. When it's possible, do it. Though I reaize it probably never is

.
Good luck!