Scratches

Sometimes I wake up with scratches on me and I don't know how they got their. The last time this happened I woke up with 5 scratches next to each other on my back. One of the scratches was deep enough to draw blood. The weirdest part about it is that the scratches are on a part of my back that I can't reach. I also checked my bed for any thing sharp that could have made the scratches. I use to have feather pillows that I blamed the scratches on but I no longer have them. Their was nothing in or around the bed that could have made the scratches. I have read that the scratches could be made by ghosts. But Its probably just aliens sticking probes inside of me. Have you ever woke up with unexplained scratches or marks? What do you think is causing them?