You could consider the Copenhagen Burnout Inventory.
http://www.researchgate.net/publication/247511197
Kristensen, T. S., Borritz, M., Villadsen, E., & Christensen, K. B. (2005). The Copenhagen Burnout Inventory: A new tool for the assessment of burnout. Work & Stress, 19, 192-207. doi:10.1080/02678370500297720
Article The Copenhagen Burnout Inventory: A new tool for the assessm...
I wrote earlier that you should consider Michael Frone's fatigue instrument. As per Renzo Bianchi's response, I also suggest that you consider using a measure of depressive symptoms. Perhaps the CES-D or a short-form version of the instrument.