How sleep shapes what we remember and forget.

I have found monitoring the quality of my sleep to be a fascinating and useful activity. I use both the Oura ring and Apple watch to monitor body temperature, body movement, heart rate, and heart rate variability, and then compare their different (but broadly similar) algorithmic estimates of deep sleep, REM sleep, non-REM sleep, and wake periods. I'm on the lookout for articles on sleep during my scans of journals' tables of contents, and have come upon this review by Sakai of what is happening in our sleep to be concise and useful. Below is a more general overview from edited and rearranged clips (The article goes into more electrophysiological and cellular details):

 

(image credit Dave Cutler)

...memory at the beginning of the consolidation process is very much anchored in hippocampal networks, and in the end of this process, it primarily resides in neocortical networks...New memories are rich with contextual clues such as the time, place, and sensory details of an experience...as memories get encoded in the cortex, many of those spatial and temporal details fade...forgetting—through weakening or loss of synapses—seems to play a key role in the process of memory consolidation, especially during sleep... What remains are the elements representing the essential core of the memory. When learning how to drive, for example, the movements needed to steer and brake are critical; the details of avoiding a specific car on a particular outing are not...sleep’s role in memory is not simply about passive storage. Rather..a more active process of consolidation that extracts key information and forms a generalized version of the overall memory that can later be accessed and applied to relevant situations.

Sleep in mammals has distinct phases as characterized by specific EEG patterns. These include brain-wide slow oscillations (less than 1 Hz in frequency), sharp-wave ripples (100-300 Hz) in the hippocampus, and spindles (10-15 Hz), which are related to the firing of neurons in the circuits connecting the thalamus and the cortex. Upon onset of sleep, the brain enters a non-rapid eye movement (non-REM) phase. During non-REM sleep, slow oscillations sweep across large regions of the brain, punctuated by swells of spindles and bursts of sharp wave-ripples. A period of rapid eye movement (REM) sleep follows, with characteristic bursts of its namesake eye movements and low-amplitude theta oscillations around 4-8 Hz. The brain cycles through these phases throughout the sleep period, with slow-wave, non-REM sleep dominating the early hours and REM sleep the late hours.

There are...

...distinct roles of different stages of sleep in memory formation. Non-REM sleep has been shown to be very important for consolidation of declarative memories—those based on recall of information—while REM sleep seems to play a larger part in procedural or task-based memories...this may relate to the degree of synaptic change required. For declarative memories, most of the foundational learning has already taken place; remembering a new fact likely requires only small changes in synaptic strengths. By contrast, procedural memories require a massive amount of synaptic change...If you want to learn how to ride a bike, or how to play capoeira … it's not like learning a new name...I’s weeks, months, years of work. And so it seems like REM sleep is really, really necessary to do this long-term persistent synaptic change.”