When you take a nap before class and dream that your arm is being attacked by Retinoic Acid and Cyp26b1 genes are deactivating the Retinoic Acid that is trying to make the bones in your arm grow shorter.
That happened to me during my quick nap. I fell asleep after spending about an hour researching bone/limb development.
Fear not. My limbs are normal size and not shorter.