it is your baby sucking for comfort, using you as a human pacifier. they're not hungry but they want to latch on.
http://kellymom.com/bf/normal/comfortnursing/ has a bit more info for you here.
Comfort latch and dream feed are similar. The child is not hungry, just using us as a human pacifier.
The child is using you to soothe himself at night instead of a pacifier.