Definitely rule out regular physical illnesses first. Anemia, mononucleosis, pregnancy (not sure of your gender), vitamin deficiencies, chronic fatigue syndrome...all of these things and more can make you feel exhausted. I'm not sure how old you are, but it could even be something as innocuous as a growth spurt. Teenagers are often very tired as their bodies grow. I think those of us with mental illnesses tend to be a little hypervigilant and overlook regular physical causes.