Blood Pressure Med Question?

I take Toprol XL for high blood pressure daily. My doctor said I could take it in the morning or evening. I tried taking it in the evening and right before bedtime, but for some reason I can't sleep when I take it after 2pm. Why would this be since it slows down your heart and body right? What is in this medicine that make falling asleep so difficult? Anybody have similar experiences with this med.