muscle spasm
"I've had several lower back issues for the majority of my life including surgery.  I used to take Flexeril but didn't like the way it made me feel.  I told my doc and he switched me to Zanaflex.  It has been a God send especially when it comes to sleep.  I used to take Ambien but has severe side effects including a dangerous sleepwalking episode.  I have replaced Flexeril and Ambien with Zanaflex with incredible results.  I take it before bed and am able to get a great nights rest without that horrible "drunk"/drowsy feeling when I get up.  The only drawback is I do get very tired approximately 30 min after taking it so I don't use it during the day."