A childhood belief about speaking

When I was little, I used to sleep in my Mom's room -- that is, the master bedroom. The door to the master bedroom was actually in the living room, so activity taking place in the living room and kitchen area could easily be heard from the master bedroom, but speech always sounded very muffled. When I was very little, probably about preschool age, I can remember waking up in my Mom's bed after my Mom and sisters were already awake, hearing their muffled conversation through the door, and concluding that my family spoke a different language in the morning -- but they only used it when I wasn't around. I'd often sit up in bed and listen to their strange "language," trying to decipher words and figure out what they meant,