Vania, I have a similar problem. I don't scratch, but very early in the morning, usually around two or three hours before I wake up, I start stretching in my sleep with my hands behind my head. I've whacked myself a head a couple times this way.

The solution I've found is to keep the room very cold at night. If a cold room doesn't bother you (I sleep better in the cold anyway), try keeping the room cold enough so that you have to keep your arms under the blankets to stay warm. I find that when I do this, I very rarely remove my arms from under the covers in my sleep.