The quantity and quality of sleep affect a person’s ability to remember, and sleep is a period where the brain consolidates memories.
Like sleep, memory is largely a mystery to scientists, although it is clear that during sleep memories are pruned and entrenched. There is a lot of research going on in this area, and the consensus is that sleep is useful (perhaps required) for consolidating memories, but not necessarily for all types of memory. The research on learning new skills and motor procedures shows sleep is required. 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. Sections of the brain – the hippocampus, neocortex and amygdala – that are important in memory are active during sleep.
Neuroscientists think that sleep enables “both quantitative and qualitative changes of memory representations.” This means that psychologists can test how people remember events and facts (qualitative) and that these memories change after sleep, and that scientific analysis of brains show changes in synaptic connections (quantitative) after sleep.
Breaking it down
Let’s consider memory in three parts: acquisition, consolidation, and recall. Acquisition and recall happen while we are awake. Consolidation happens during waking and sleep. Consolidation means moving the memory from a short-term “buffer-like” memory to a long-term memory and updating believes and general knowledge with new learning.
Memories appear to be cemented and formed in all three types of sleep – light sleep, deep sleep, and REM sleep. There is conflicting evidence about memory formation during REM and what kind of memory formation happens may be qualitatively different from what happens in NREM sleep.
Selecting What to Remember
As scientists attempt to uncover the electrophysiological mechanisms of how memory is stored and retrieved, a term seen in the literature is “cherry-picking”. This is how scientists describe the brain’s process of selecting memories and making them long-lasting during sleep.
Scientists believe that the hippocampus and neocortex use different methods to store memories. The hippocampus stores unique representations useful for episodic memory while the neocortex allows overlapping representations useful for understanding patterns. This is semantic memory. The short-term memory is thought to be encoded as patterns of neural activity while long-term memories are structural changes in the brain – the formation of new synapses that are more persistent. This explains why short-term amnesia can happen (memory from the last 12 hours or so is wiped) without affecting long-term memories.
“Consolidation” means some neural circuits are strengthened and others are erased and or let go so that new memories can form.
Scholars Ken A. Paller and Joel L. 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. Declarative memory is defined here as the ability to recall specific facts and events, as opposed to background knowledge and emotions. And that when it comes to an individual’s understanding of the world “sleep is essentially a nightly session of psychotherapy” in the words of these Northwestern University scientists (This analogy may not be perfect.)
The communication between the hippocampus and neocortex allows new data learned the previous day to “update” understanding in the neocortex. Electrophysiological, computational and neuroimaging studies have shown information transfers to the neocortex. The schematized versions of some of the short-term memories contribute to learning so that even when memory of a specific event is gone, that event still contributed to overall knowledge.
Particularly during stage 3 deep sleep, the memories that have been put in the hippocampus (short-term memories formed during the previous day) are redistributed to the neocortex (where they will be long-term memories).
This transfer of information happens during waking hours, too, as well as NREM sleep. During REM sleep there is not much communication between these two areas of the brain. Instead, the neocortex replays memories to itself.
This model of memory consolidation during sleep explains why abnormal NREM sleep – reduced slow-wave sleep and during Stage 2 – is tied to problems in forming memories for patients with Alzheimer’s, schizophrenia, and fibromyalgia.
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. 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. 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). Rats set to learn their way around a complicated maze had a longer REM period when allowed to sleep. Was this due to increased demands on memory formation or because the rats were stressed from the new environment? It is not clear.
There is also conflicting evidence on how sleep deprivation affects our ability to remember. Experiments on animals that were deprived of REM show they cannot learn new things as well. It is not clear whether this reduction in capabilities is due to a lack of REM in particular or an overall tiredness in the animals.
Neurochemistry and Conscious Experience
High levels of cortisol may disrupt the transfer of information between the hippocampus and neocortex. This can actually change the content of dreams as subjectively experienced, and it may explain why people experiencing stress (and high cortisol levels) don’t learn difficult conceptual material as readily.
The sleeping person experiences this memory transfer and consolidation at least partly as dreams. The content of dreams is beyond the scope of Sleepdex (we put no stock in “dream interpretation”), but it is clear that the person’s activities the previous day play a big part in what the dreams are about. 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. Of course, caffeine is a way to mitigate the effects of adenosine, which may explain why the students drink so much coffee and energy drinks. Tests with rats found that interrupted sleep made new memory formation difficult, which may have implications for the many people who experience sleep interruptions due to alcohol consumption-induced fragmentation and apnea.
There is also evidence that prescription sleeping pills inhibit the consolidation of memory. Tests with zolpidem and zaleplon found that people had trouble remembering things learned the previous day when they took the drugs.
What if a person experiences a particularly memorable day? One in which he or she is exposed to many new stimuli. Does this mean the brain needs to sleep longer to accommodate more memories than normal? It has been proposed that sleep after a novel day is more intense, but this has yet to be definitively proven. We do not know of any studies that validate this hypothesis.
Other experiments suggest that sleep deprivation does not affect the visual working memory but it does affect the ability of the mind to filter out distractions and pay attention. Sleep deprivation therefore makes it hard to concentrate and learn. Experiments on students learning academic material found memory was better when study sessions were spaced out over time. The sleep periods (nightly sleep) is thought to consolidate those memories.