What Causes Trouble Sleeping While On Beta Blockers?

I was put on metoprolol 25mg QD on Jan. 7, 2015 for tachycardia. The doctor suspects hypothyroidism and possibly Hashimoto's disease through lab work, and I am scheduled for an US soon. Anyway, since on this beta blocker, I am having problems sleeping, and have slept very little since 1/7, maybe 3 hours average. I have a script for Ristoril 25mg and Xanax 1 mg for anxiety, and I did try it, but nothing seems to be helping. Any advice?