Siri is randomly speaking in the middle of the night, multiple times.

I have an iPhone 5s and Siri has started to come on randomly in the middle of the night. Its normally the "I didn't understand you messge", but not always. Sometimes I just too tired to realize what she said and just want her to shut up. It started a few weeks ago and would happen maybe once a week. It happened 7 times last night! I lock my phone as well as leave it on a bed side table when I go to sleep. I don't want to disable Siri since I do use it, but I really need sleep!