Why Do Muscle Relaxers make you SLEEP!

Wow I have been out sleeping for 2 days now. My new found leg pain has nothing to do with my back :-) I have a Rt. Quadricept Strain and will out of work for at least 3 weeks! How do you get and injury to your quadricept. I am no athlete by any means. I start physical therapy on Tuesday. Hope all goes well and that I have a fast recovery. If I could stay awake, I could get some much needed computer work done!! Bonnie:-)