It's because at 37.5mg you are only getting the serotonin reuptake effect and I had the same problem at 75mg with it making me tired. 150mg and above is really where it becomes more energizing and effective because this is where norepinephrine kicks in.
I would try and see if you can get your doc to get you upto 150mg as I think this will significantly help.
Disorders: Dysthymic Disorder, Social Anxiety Disorder, Generalized Anxiety Disorder, Panic Disorder.
Medications: Propranalol, Valium, Prozac, Celexa, Paxil, Zoloft, Roboxetine, Effexor, Mirtazapine.
Current status: Depressed and looking for a solution.