But even people with injuries and medications that limit REM sleep do not report memory problems and sleep expert Jerome Siegel has been quoted about people with brain injuries who do not go through REM sleep and nevertheless find a way to function normally.
More recent investigation casts doubt on those earlier assertions.
The quantity and quality of sleep affect a person’s ability to remember, and sleep is a period where the brain consolidates memories.
Early scientific work found REM sleep does not appear to have a major part in consolidation of memories.
This may sound surprising as REM is closer to waking than any other phase of sleep.
For “declarative memory” sleep also seems critical to long-term retention.
For simpler conditioning – relating associations between stimuli or a response to a stimulus, the evidence is not as strong, but sleep still seems beneficial for that type of memory.
It is also known that dreams in NREM sleep are fragmentary, while REM dreams are more often coherent and “cinematic”.
Researchers have identified adenosine build-up as a reason sleep deprivation affects memory formation.
EEG studies find that after a cognitively taxing day, length of REM periods often increase, suggesting that the brain is “resting” from learning during REM.
The increase in REM appears most strikingly after the mind is asked to acquire more declarative memory (remember more facts).
This explains why short-term amnesia can happen (memory from the last 12 hours or so is wiped) without affecting long-term memories. Voss wrote an article in 2004 laying out their hypothesis that pruning of available memory happens at night as the brain shuffles and adopts what it will put into declarative memory.