Question: If sleep helps you prepare memories for future use then why do you forget some memories in the future?

Alex Reid answered on 13 Nov 2018: last edited 13 Nov 2018 10:03 am

Hi Rice and Curry, this is a brilliant question! To answer it properly I need to address it in two parts: 1. why forgetting things helps memory in the future, and; 2 what sleep does to help do this. So we forget a lot of things naturally but in several ways it might also be a deliberate thing our brain does to help optimise itself. Firstly, at an emotional level, if we remembered every trauma and negative thing that ever happened to us in our lives in fine detail that might be mentally unhealthy, as seen in disorders such as post traumatic stress disorder (PTSD). Secondly, some memories might become redundant over time and if we kept them they would interfere with memories you need to remember. For example, if you have to change the password on your computer it might be good to forget the old one so you don’t keep accidentally typing it in! Thirdly, not all memories are equally important, and in promoting some more than others your memory becomes better tuned to the things that matter. Lastly, in terms of future use, our brain is not just a head camera recording life, it is interested in doing things. As such it may want to convert memories into a form that help us to do that, and some parts of certain memories may get dropped in this process.
The area that I look at, sleep, has been observed helping all of the above. Sleep has a close relationship with emotional memory and mental health. It can take negative memories and reduce your emotional response to them while keeping the potentially important information contained in the memory. It is a bit like unpeeling an orange, if the orange rind was the associated emotional response, and the orange segments were the memory you want to keep. Sleep as also been observed to negate (remove) the effects of memory interference from older (and less useful) memories. Additionally, sleep promotes memories that are of particular importance, and gives priority to memories based on a number of factors (such as reward and personal relevance). Lastly, sleep helps convert one memory form to another. Your memory of specific events in time is known as episodic memory, but we also have a type of memory called semantic memory, which is your memory of concepts. For example, you may not need to remember every specific instance of when you had breakfast and what you ate every time (episodic memory) but you do need to maintain and update your concept of what a breakfast is (semantic memory). Sleep can help convert individual breakfast instances into your concept of a breakfast. That way you lose the irrelevant details but your concept keeps getting updated and becomes better at predicting what a future breakfast might involve.