Sleep Amnesia

There is a reason you don’t remember falling asleep. Falling asleep is a very smooth transition. You make a transition from awake to pre-sleep, light sleep, slow-wave sleep, and then to REM sleep. Memory formation depends heavily on changes. We are most likely to remember something new rather than something that doesn't really alter over time. Falling asleep is a rather slow change so there is limited need to remember it.

Secondly, pre-sleep goes hand in hand with decreased perceptual awareness. With decreased awareness, there is just less content to store. With the decreased perceptual awareness we switch to an internal reflective state, thinking of things rather than learning new things. So you start to think about other things and then there is nothing new to remember.