Here's a guess: it's not just the long hours at work, but the sum of all the stress and drain of full-time schooling added to the repetition of the physical labor. As a long-distance cyclist, you may be used to pushing yourself on grueling rides--but in your broader life, when do you recharge? Surely you are aware of the need for recovery--physically, mentally, emotionally--and take some days off from riding. Between cycling, work and school, you may feel so exhausted because you don't allow yourself time to build up those stores of energy?

From your recent thread about teaching overseas, am I correct that you've just recently gone back to school? Maybe you are still figuring out how to balance riding and work with the newly-added demands of school.