anxiety
"I suffer from panic attacks that occur after I go to sleep.  I wake up about an hour or so after I fall asleep (I don't have trouble falling asleep) freaking out.  My heart is racing, I am terrified and I feel like I will go crazy before it stops.  They are so scary because no matter how many times I have suffered through them, I am always sure I am dying when I have one.  Previously I was given other medications like Buspar or Paxil, but I didn't tolerate them very well and they did not stop the attacks - only slowed them down.  Once I was prescribed Xanax I noticed that the medication would actually stop a panic attack in its tracks, which was a miracle.  The medication has almost completely stopped the attacks - but not 100 %."