What Causes Fatigue While Taking Melatonin?

I ve been taking 10mg of melatonin along with tizanidine at night to help get me to sleep as a treatment of DSPS for about a month now. The past few days I ve been experiencing extreme fatigue and having trouble moving around because my body just doesn t seem to want to move. Could I be getting a buildup of melatonin? What would you suggest?