What Causes Tongue Biting In The Night?

Two years ago I woke up and realized that I had bit my tongue during the night there was blood on my pillow and it happened now again and Then I couldnt remember certain things. I went fopr MR scans,EEG and several blood tests and they could find nothing wrong> Some doctors say its an epiletic attack others say it is a brain spasm.Is it the same thing and what is it?