Being an Angel, there were a lot of things Castiel knew. However, Sign Language was not one of them.

He knew it must've been hard to be deaf, only being able to communicate through paper and Sam’s rusty ASL knowledge, so Cas decided to learn.

Since he didn't sleep, and he had a very good memory, it only took a few weeks until he was very well-versed in the Language.

He waited until you woke up, to approach you. He watched as you entered the kitchen, going to the fridge for water.