[Brandye]

Each of us is slightly different in our specific anatomy. But, look at one of those cutaway diagrams of the female reproductive system and imagine it "on its' back." It is possible for the blood to pool significantly if one does not move much during the night. Remember that the entire flow is only a few tablespoonfuls, so one nights'worth is not that much - even though it seems like gallons. Stand the diagram up again and things start to flow.

Another possibility is that you sleep in a position that may actually crimp the uterus just above the cervix or with the cervical os pressing against the vaginal wall. Again, the flow will be temporarily stopped. Untangle yourself, and you flow again.