Memory and sleep- sleep can contribute to memory, recovery, and so much more – the mystical world of sleep and where we go when we do rest is so fascinating to scientists. There is an abundance of research on the sleep cycles, and recently we covered off some of the following topics on sleep:

• Melatonin.
• Poor sleep and weight gain.
• Sleep states, and stress.

But, this article is going a little deeper into the knowledge of sleep, and that is on memory consolidation. How what we take in during the day is consolidated into our memory banks, where we store what and what sleep stages dictate how we develop certain forms of memory.

The Stages of Sleep

Sleep can be defined between 2 overarching stages generally which are, REM (Rapid Eye Movement) and Non-REM – which is every other phase excluding that of REM. Outside of REM there are 3 sub-stages that occur during Non-REM, these include:

• Stage 1 – This occurs during usually the very first stages of sleep when we drift off. Alpha waves (8-12hz) that keep us alert and in the now begin to dissipate and Theta Waves (3-8hz) start to take front stage. These, as we will find out later, are associated with learning, memory, and intuition, they are deep meditating slow range waves.
• Stage 2 – Theta Waves are in full swing. There is zero eye movement in this stage, but is the most active phase for what is called 'sleep spindles', these are short and sharp bursts of high-frequency brain activity, around 9-12hz for slow and 12-15hz for fast, they last for 1- 1.5 sec bursts. Originating from thalamic and thalamocortical neurons, we will cover this more below, but their frequency occurs more in the evening after a day spent in ‘new learning mode’. This can account for both cognitive, and motor skills learned that day, with higher retention rates coinciding the following day after high spindle activity[1].
• Stage 3 – Otherwise known commonly as deep sleep to most. It is a slow wave state where the delta waves (0.5 - 2hz) take hold, this is another dreaming state, although not as active as REM for dreams. Dreams that occur in slow-wave sleep or deep sleep as we call it, are far less vivid, more disconnected, and fragmented. We tend to hold no recollection of them as they don’t depict something that makes constructive sense.

Deeper Insight into ‘Sleep Spindles’

Covering sleep spindles briefly above, we will break them down some more here as they are important in the grand scheme of memory consolidation. Sleep spindles are titled as such because of their coil-like structure that stands out on an EEG (electroencephalogram) – a test that detects electrical activity of the brain. The spindles look like coiled bursts on an EEG of fast waves in amongst a relatively slow wave state. They occur as we mentioned in stage 2 of Non-REM sleep. Their purpose, from what we know, is around the consolidative function of memories for new learning that occurred during that same day. Both cognitive and motor skills appear remarkably improved the following day, after a night where high spindle activity occurred[2,3,4].

Thalamocortical spindles oscillate between that of 10-15hz and last anywhere from 1-1.5 seconds at a time. Sleep deprivation studies after a day of learning were conducted to test the regular occurring confirmations that sleep spindles play a role in memory consolidation and show staggering interference results on subjects around the recall and retention of information learned in both cognitive and motor skills[5,6]. They arise from the interaction of both the thalamic and cortical neurons in the brain and occur rhythmically every 10 seconds on average for each stage 2 cycle. The stage 2 cycle usually lasts for 20-25 mins before moving into the next stage. There are both fast and slow spindles, slow spindles are usually observed in the frontal regions of the brain, and faster spindles are often observed in the parietal region[7].

What is ‘Memory Consolidation’?

Memory and learned behavior have been at the forefront of discussion for as long as humans have posed this question. How is it that we learn and consolidate skills? Pavlov in 1927 famously suggested that spreading activation between nodes was necessary for memory formation[8]. Similarly, in 1949, Donald Hebb proposed that experiencing a new learning event activates specific neural circuits, that while active; strengthen the synaptic connections that constitute the basis for that experience to be later stored as memory[9].

Put simply; memory consolidation refers to the process by which temporary memory is transformed into a more stable, long-lasting form that is able to be recalled long after it was created and experienced. Lechner in 1999 studied this in-depth and more specifically around the phenomenon of 'retroactive interference', that is where new information seemingly interrupts the recall of old news, and this can occur for a period of time after the learning has occurred[10,11]. His notable observation here was that more recent memories were highly vulnerable to disease or injury than that of remote, distant memories, which laid the groundwork for the distinguishing difference between learning in the moment and the process in which long term consolidation is built upon.

Memory consolidation as we know it today is dependant on information stored in both the hippocampus in our brain and also the neocortex. Consolidation of the memory from this is the process in which the hippocampus guides the structure of temporary memory and the reorganization of the information initially stored, across into the neocortex and moves it away independently from the hippocampus, supporting the formation of the memory indefinitely[12].

How Sleep Forms our Memories

The transfer of information from the hippocampus in the brain to the neocortex for long term storage occurs indicative of the oscillating activity. Patterns of slow oscillations as we discussed earlier that occur in stage two Non-REM in the cortex, are influenced by the hippocampal sharp-wave ripples. Hippocampal sharp-wave ripples are an event in the brain associated with highly synchronized neural firing activity that occurs in the hippocampus and modulates neural activity in various brain regions. These patterns of slow oscillations determine the synaptic changes in the cortex.

Changes that then occur in synaptic strength are widely believed to underlie learning and memory storage in the brain. Sharp wave ripples are supposed to support memory consolidation from short to long-term memory storage but also retrieval processes required for decision making in the mammalian brain[13]. Slow-wave sleep is generally deemed then as one of the essential stages of non-rem sleep in which memory consolidation and neuroplasticity occurs most. Stage 2 Non-Rem sleep lasts for cycles of 20-25 mins and repeats throughout our full rest; stage 2 Non-Rem contributes to on average 50% of our total sleep.

What about REM Sleep?

We mentioned REM Sleep early on in the piece, REM or Rapid Eye Movement sleep is fascinating, to say the least. A lot happens here in REM. We have more vivid and intense dreams, eye movement is rapid, and our body undergoes a state of muscle paralysis, so we protect ourselves from acting out or defending ourselves from our dreams, it's wild! The duration of REM dips from around 10 mins in the first cycle up to 50 min for a cycle towards the end stages of our sleep entirety. On an EEG our brain waves look eerily similar to that of the ones we have when we are awake, alert, and attentive to the outside world, except that we are paralyzed and ... asleep.

Interestingly, babies have far more REM sleep than adults; they can have anywhere up to 8 hours, whereas adults in their 20's can have a total of 2 hours of REM and, as we age it lessens again to about 45 Mins. Why is this? REM is actually associated with brain volume development in proportion to that of their body mass and as such can be used as an indicative indicator of neurodevelopmental differences between individuals of the same species[14]. This may suggest why infants often require 14-16 hours of sleep per day if 8 of those are spent in massive amounts of REM state. Let sleeping babies lie rings true now more than ever.

In adults notably we need less, so what do we use REM For? REM is important in the process of memory consolidation around visual input, neural maturation, and synaptic plasticity in the optical system, which may be why we have more vivid dreams in this state than others.

REM and N-REM Memory Consolidation Re-cap

REM sleep is crucial to our visual input development and synaptic plasticity in the visual regions as well as that of our non-declarative memory. Both REM and NREM contribute to non-declarative/implicit memory consolidation which is like automated memory over time; we do not have to recall the skill once it has been mastered, like riding a bike for example. You can jump on a bike in 20 years and still know what is required.

NREM is where sleep spindle activity originating in the hippocampus during slow-wave sleep may serve the function of memory consolidation by transferring information from the hippocampus to the neocortex, further again, that following the transfer of data from the hippocampus and amygdala in the brain to the neocortex during NREM sleep; it will also then later be processed for consolidation in REM for what may be relevant in this state[15]. NREM is uniquely associated with that of 'motor skill' memory development. Declarative memory can develop across both stages, but can be damaged and fragmented if the cycles of REM and NREM are disrupted; so are dependent on each other for this to occur[16]. Declarative/explicit memory being that of which can be consciously recalled like an event or known fact, it can be essentially ‘declared’.

Which is More Important?

They are equally important – you should be getting diversity in cycles across your 8 hours. Sleep deprivation will take from both REM and NREM capacity, you cannot favor one over the other in that instance, and deep troughs of sleep are required for a feeling of rest and essential to recovery.
Unwinding and having a good sleep hygiene routine is best to ensure you are getting the diversity in this state of unconsciousness. Try to unplug around an hour before bed and switch the phone to a book if you want something to help you fall asleep, some memory retention from the book you are reading is likely far more beneficial than scrolling through social media until you can't keep your eyes open any longer.

Ensure you have enough water before bed, during sleep we enter the glymphatic movement where the cerebral spinal fluid takes up a larger volume in the subarachnoid space between the brain and skull to flush and remove waste and toxin build-up out of the brain and into the lymphatics for removal upon waking and beginning movement of the body. The cerebral spinal fluid consists of 99% water, and there exists around 150ml of it in the body[17].

Be consistent, one night of good sleep does not equate to a catch up of six nights of poor sleep. You will never regain lost sleep, it's accumulative, so if anything, be consistent, set a time to go by, and wake up at a similar time each day – it will help with your circadian rhythms and internal driving clock, our Suprachiasmatic Nucleus . We sleep 1/3 of our lives, but it dictates how we live and experience the other 2/3, be wise with it.
References

1. https://www.goodnights.rest/the-science-of-sleep/what-are-sleep-spindles/
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10. Alves, Marcus Vinicius & Bueno, Orlando. (2017). Retroactive Interference: Forgetting as an Interruption of Memory Consolidation. Temas em Psicologia. 25. 1055-1067. 10.9788/TP2017.3-07En.
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13. Joo, H.R., Frank, L.M. The hippocampal sharp wave–ripple in memory retrieval for immediate use and consolidation. Nat Rev Neurosci 19, 744–757 (2018). https://doi.org/10.1038/s41583-018-0077-1
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