How does sleep impact the formation of memories?

There’s this theory, probably the one that most people in the sleep field support, called the synaptic homeostasis hypothesis. It’s this idea that during the day, we build all these excitatory connections and associations between things that do or do not totally matter to your wellbeing and survival. During sleep, in deep sleep, you down-regulate all of those memories that happen during the day so that things that are more salient to your survival rise to the top. It used to be, oh, the predators in the jungle are in that area, don’t go over there. Now it’s like, oh, what did Mindy say about me on social media? All the things that are relevant to your personality and wellbeing get preferential treatment during deep sleep and then you replay and integrate that information during REM.