Facetime freezing, affecting sleep mode on imac

I downloaded facetime several months ago and haven't used it til today. Opened it to familiarize myself with it before a call later today. It froze. Then my imac went to sleep. Woke imac up and closed and reopened facetime. It froze so I reopened it. It seemed okay. But now, when I try to put my mac in sleep mode, it won't go to sleep right away, takes about 30 or more seconds. Any thoughts on why facetime is freezing and why it would affect my mac's sleep mode?