The unconscious mind
The first is the conscious mind. This is the part of your mind you notice. When you make the decision to go to the store, you are using your conscious mind. When you put milk and eggs into your cart, you are using your conscious mind.
The second is the unconscious mind. The unconscious mind regulates your heartbeat and keeps you breathing even when you are not thinking about it. It does not need instructions because it knows what to do without conscious involvement.
When you are going to sleep, you are surrendering your body to the unconscious mind so that it may rest and rejuvenate your body. And while you cannot directly control your unconscious mind, you are able to communicate with it. You are able to communicate to it that you wish to go to sleep and that it has the go-ahead to take over.