Deep Non-Rem Sleep and Memory Replay in the Human Brain

The final installment of this series focuses on what happens inside your brain during deep, non-REM sleep that allows you to fixate new memories. There are at least two mechanisms at work here. One is a process of shifting information from short to long-term memory. Each night when you go into deep non REM sleep, those long range brainwaves of deep sleep are going to be moving packets of information, from that short term, vulnerable, USB stick-like memory reservoir (the hippocampus) to the safer long term storage site (the cortex). When we wake up each morning, we've now cleared out that USB memory stick, and we've refreshed and restored our ability to start learning new memories. This process repeats, filling up the USB stick with new information during the day, and then at night, transferring those memories over to the long term storage site in the cortex.

The second memory mechanism, memory replay, is very different. Back in the 1990s, scientists recording the activity of individual brain cells in the memory centers of rats running around a maze discovered that different brain cells coded different parts of the maze. More remarkable was the discovery that the sleeping brain would replay these new memory sequences during deep, non-REM sleep much faster than standard waking speeds. This meant the memory trace was being replayed many times, and the brain was repeatedly etching it more permanently into the brain.

There is at least one other time during sleep when we see memory replay—REM sleep, the stage when we dream. But here, memories are replayed at much slower speeds, sometimes just half the speed of waking experience. Because of this temporal distortion - where time within the dream is slowed down and expanded - you will feel as though you have been dreaming for far longer than you actually have. The movie Inception, in fact, draws some inspiration from this concept - almost as though the director and the writer of the movie Chris Nolan had a little bit of idea help from a sleep scientist!

Please note that Matt is not a medical doctor, and none of the content in this podcast should be considered medical advice in any way, shape, or form, nor prescriptive in any way.

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