How can we? The instant that we doze off into sleep, there's a little period of time before that when we are half-asleep and begin to lose sense of our surroundings. This is the time period that we don't remember simply because we are not alert and aware of our surroundings.
If someone were to wake you up like 10 minutes after you have fully dozed off and told you to recall the time before you slept, you would be able to recall the time before the semi-sleep period and nothing after that.
That's how I think it works.
