Speak a language that never known

This year, in midnight, when I am in between sleeping and awaking, I often speak out a language that I don't understand what I am talking about, it is not English, I don't know what 'language' it is. I can stop from speaking, but I feel comfortable if I keep on speaking. I can keep on speaking for 20 mins without repeating. I don't have this 'functionality' during the daytime. Do I get mental problem? What is the reason behind?