Woke up fine now I'm tired

This morning was great, I had some reading to do and when I got up at 9:30 I was ready to begin the day. I sat up in bed and felt rejuvenated for the day ahead. As the day wore on I still was doing fine, but as soon as it got to my first class of the day at 3:20 I got tired and could hardly keep my eyes open in class. It was so confusing to me, I had woke up with such vigor and now could hardly stay awake for a class that was an hour long. The thing that always makes me feel bad is that as soon as class is over, I get over it. I feel better and am ready for more activities. It happened this way today until I tried to read my textbooks for class tomorrow. I was great, collapsed in class and then I was fine again until more school work came and then I couldn't hold my eyelids open. School work just doesn't hold enough excitement to get the adrenaline pumping and keep the wear and tear of the day off. Who would have guessed. I also realize that this is most likely a first glimpse at what is to come for me throughout the semester.