Dogs again

Why is it that about once a week my dogs will fight in the middle of the night. My theory is that the big dog has nightmares and attacks the little dog while she is sleeping. It does seem to go this way. It's always the big dog who attacks, and then looks sheepish when I have to go and break them up. If I put them in separate rooms, the little dog frets and won't go to sleep. She has never slept alone in her life. Instead, she scrabbles at the door and lets out her famous Toller cry, which is really more of a yodelling nose than anything else. People are stunned when they hear it. I'm just worried that at some stage the big dog might really hurt the little one, as she's twice her size. Any advice from the dog owners out there?