>>12736890To answer this, there is an area in your brain, hippocampus, that is responsible for keeping all the memories and experiences and senses from a current day, the day before you went to sleep. And this has a limited amount of memory and thus when you sleep, and are in your NREM phase your brain slowly processes the memory and decides which memory is more important and which is not and moves it from the hippocampus. Thus when you start your day after sleep your hippocampus is ready to use again. Also there are other processes which remove distant memories from already stored places, memories of things that you do not remember often, for example a shirt you saw someone wear 5 years ago and thus make space free for other memory. If this wouldn't happen you wouldn't memorize well current tasks but would remember what shirt did your friend wear 5 years ago. Evolutionary consequences of NREM phase not cleaning your distant memories and reshaping new ones are drastical, for example: a hunter who had remembered what he ate 5 years ago couldn't survive for 5 seconds in the hunter-gathering era.
