Episode 8: The Neuroscience of Sleep, Memory and Really Good Afternoon Naps

We spend a third of our lives sleeping and there is no greater enjoyment than a really good afternoon nap. Back in preschool, the good old days, naps were built into our daily schedules but as adults - not so much. But should they be? Is there any neurological benefit or detriment to taking a nap?

Tune in to learn a little bit more about the neuroscience behind sleep, memory, and naps!

Please rate, review, and subscribe and if you have any questions, comments, concerns, queries, or complaints, please email me at neuroscienceamateurhour@gmail.com or DM me at NeuroscienceAmateurHour on Instagram. 

Citations and relevant papers below:

Cherry K. The 4 Stages of Sleep (NREM and REM Sleep Cycles). Verywell Health. Published June 16, 2007. https://www.verywellhealth.com/the-four-stages-of-sleep-2795920

The Haunting Effects Of Going Days Without Sleep. NPR.org. https://www.npr.org/2017/12/27/573739653/the-haunting-effects-of-going-days-without-sleep#:~:text=VEDANTAM%3A%20At%202%3A00%20in..

Alberini CM, Chen DY. Memory enhancement: consolidation, reconsolidation and insulin-like growth factor 2. Trends in Neurosciences. 2012;35(5):274-283. doi:10.1016/j.tins.2011.12.007

Walker MP, Stickgold R. Sleep-Dependent Learning and Memory Consolidation. Neuron. 2004;44(1):121-133. doi:10.1016/j.neuron.2004.08.031

McKenzie S, Eichenbaum H. Consolidation and Reconsolidation: Two Lives of Memories? Neuron. 2011;71(2):224-233. doi:10.1016/j.neuron.2011.06.037

Rasch B, Born J. About Sleep’s Role in Memory. Physiological Reviews. 2013;93(2):681-766. doi:10.1152/physrev.00032.2012

Walker MP. The role of slow wave sleep in memory processing. Journal of clinical sleep medicine : JCSM : official publication of the American Academy of Sleep Medicine. 2009;5(2 Suppl):S20-6. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2824214/

Kumar D, Koyanagi I, Carrier-Ruiz A, et al. Sparse Activity of Hippocampal Adult-Born Neurons during REM Sleep Is Necessary for Memory Consolidation. Neuron. 2020;107(3):552-565.e10. doi:10.1016/j.neuron.2020.05.008

Groch S, Wilhelm I, Diekelmann S, Born J. The role of REM sleep in the processing of emotional memories: Evidence from behavior and event-related potentials. Neurobiology of Learning and Memory. 2013;99:1-9. doi:10.1016/j.nlm.2012.10.006

Horváth K, Plunkett K. Spotlight on daytime napping during early childhood. Nature and Science of Sleep. 2018;Volume 10:97-104. doi:10.2147/nss.s126252

Transcript

Hello! My name is Barbara and this is Neuroscience: Amateur Hour: 

*intro music*

Thank you so much for tuning back in! I’m so excited that you’re here!

Today I want to dive into the neuroscience of sleep, memory, and really good afternoon naps. 

This episode is inspired by my all-time favorite room at work which is the confocal microscopy room. It is small, windowless, dark, and isolated. I bring my little space heater in there and get all cozy. My coworker calls it my cave lol. Despite usually being hyped on 3-6 cups of coffee, being in the confocal room always makes me want to take some mid-day naps. 

I think we can all remember our pre-school years when naps were built into our schedules in the middle of the day. The good old days. As an adult - not so much? But what if they were? Is there any neurological benefit or detriment to taking naps in the first place? 

In broad terms - some studies have shown that in addition to reducing daytime sleepiness, naps can provide a number of neurological benefits including memory consolidation, preparation for more learning (put these in high schools guys), executive functioning enhancement, and a boost in emotional stability. 

However, the other story is that increased napping has been associated with cognitive decline, hypertension, diabetes, etc. 

Shall we find out why this is? I say yes. 

So let’s dive straight into the neuroscience of sleep and memory. You may have heard of the sleep cycle, the idea that sleep is composed of multiple, physiologically unique stages. As you drift off, you enter nonrem sleep 1. Here, your body starts to relax and the muscles twitch, everything generally kinda slows down. The brain is still fairly active, producing high amplitude theta waves, slow brain waves occurring primarily in the frontal part of the brain. 

Just as a quick clarification because brain waves have always been a confusing point for me - your brain is constantly producing bursts of electrical activity and different patterns and frequencies of these electrical pulses produce different kinds of brain waves which are associated with different aspects of cognition and brain state. 

From there, you move into nonrem sleep 2 where the body temperature begins to drop and the heart rate slows. This stage takes up about 50% of the average sleep cycle and lasts about 20 min. The brain begins to produce bursts of rapid, rhythmic brain activity called sleep spindles.

Now that everything is slowed down, your body drifts off into deep deep sleep, non rem sleep stage 3, otherwise known as delta sleep because during this stage, the slowest brain waves - delta waves begin to emerge. 

Finally, your body transitions into REM which stands for rapid eye movement. In this stage, your body is immobilized but your brain is on fire. It lights up with activity and your eyes move rapidly. This is when you dream. So it kinda makes sense that your body is paralyzed so you don’t actually start acting out all the crazy shit you dream about. 

During sleep, the body shifts between sleep stages, beginning with NREM 1, moving into NREM 2, then NREM 3, and then again by NREM 2 and then by REM, with these cycles repeating over and over again, each cycle lasting about 90-110 minutes long. 

We know that sleep is vital. In fact, we can’t survive without sleep. The longest anyone has gone without sleeping and not dying is 11 days, a record held by a teenager, Randy Gardner who decided to break the previous world record for his high school science fair and thankfully for his efforts won first prize and a fun trip to the local naval base to get his brain waves measured! 

We don’t know why we need sleep to survive but we do know that after just 24 hours of sleep deprivation, the effects on the body and mind are horrible. Sleep is important to develop correctly, conserve energy when we need to, clear waste from the brain, modulate our immune systems, and many more functions. But one of the most important functions and one that is a very active area of research is the role of sleep in memory consolidation. 

So how do memories form? We can begin to form memory by interacting with some sort of external stimulus - a lecture in class, an interaction with a friend, or a shitty date. That’s acquisition. 

Following the acquisition, the memory representation undergoes several subsequent stages of development, commonly known as consolidation. 

Consolidation is the idea that the memory becomes stable in your brain, resistant to interference from competing or disruptive factors. Importantly, it also becomes long-lasting and can be called up at a later time. Memory consolidation is associated with specific gene expression and changes in the neuron networks, whose interruption can result in memory loss. 

Once the memories are consolidated, they can be retrieved, remembered when you need them to - such as lecture material for a test or when you're on your next Tinder date and need to remember how bad the last one went. Memories then undergo another process called reconsolidation, a way of updating your memories and your brain’s way of storing them. Effectively, reconsolidating your memories allows your brain to reorganize your memory banks and incorporate new knowledge that you’ve acquired. 

In 1885, Ebbinghaus, the father of experimental memory research, published a series of studies he had conducted on himself about forgetting lists of nonsense-word pairs that established the now well-known forgetting curve - the idea that memory retention declines over time. He observed that forgetting was reduced when he slept during the retention interval. Further studies found that depriving someone of sleep impaired that person’s ability to remember things. 

These findings brought about the idea that forgetting something is not so much a matter of decay of old impressions and associations as it is a matter of interference, inhibition, or obliteration of old memories by new ones. 

Sleep may represent a time period in which new encoding of external or internal stimulus is greatly reduced, barring any interferences and facilitating memory retention. 

As researchers learned more about sleep, they sought to investigate whether different aspects of memory consolidation are functionally related to different sleep stages. 

Spoiler alert - they are!

Focus started on REM sleep as a key phase during which memory consolidation occurs due to the presence of vibrant dreams and individuals remembering those dreams. Studies in animals corroborated those findings at the time, showing that there appeared to be an increase in REM sleep after learning tasks. 

One theory was that REM sleep may be involved in the erasure or filtering of information. One research group, using a neurocomputational model of associative learning, suggested that dreaming reduced unwanted and bizarre forms of representation in memory, which may have enhanced learning the next day as well as the retrieval of memories acquired before sleep. 

However, more recent studies have not found much evidence in support of that theory. In one study, volunteers were asked to voluntarily suppress “unwanted memories” but sleep and particularly REM sleep appeared to counteract this suppression, rather than enhancing the forgetting of “unwanted memories”. 

In addition, experimental design appears to be flawed in early works. The way that animals are restricted from going into REM sleep is that they are placed on a small platform surrounded by water and when REM sleep occurs and the aforementioned muscular paralysis begins, the animal falls in the water. This kind of awakening could be incredibly traumatic and may have had some sort of impact on memory retention beyond the lack of REM sleep. 

But given the importance of sleep, researchers have continued to try and find a link between REM sleep and memory consolidation. A more recent study out of Japan found that sparse activity of hippocampal adult-born neurons (a rare kind of brain cell that develops when you’re already an adult as opposed to when you’re developing) - these neurons, specifically during REM sleep, are necessary for memory consolidation. These adult-born neurons, which were active during learning a contextual fear paradigm reactivate during REM sleep, perhaps aiding in the flow of memories and information from one brain region to another for long-term consolidation. 

Further, another important finding is REM sleep may be vital in strengthening and modulating emotional memories. One study had sixteen young men learn 50 neutral pictures and 50 negative pictures before falling asleep into either slow-wave sleep rich sleep or REM rich sleep. They found that recognition was better for the emotional pictures than the neutral pictures after REM compared to slow-wave sleep. REM sleep may represent a unique brain state that allows emotionally modulated integration of memory traces that had been previously consolidated. It may also be responsible for the disengagement of successfully consolidated memory traces from the hippocampus so that the information can flow to another brain region for later retrieval.  

But as focus shifts away from REM sleep and memory consolidation, it finds itself focused anew on the other stages, specifically slow-wave sleep, otherwise known as deep, restorative sleep, or NREM sleep. This stage of sleep is primarily characterized by delta waves and has been shown to be important for the reinforcement of declarative memory. -  memory devoted to facts - names, dates, events, and so on. 

Multiple studies have shown that with increased slow-wave sleep, there was a significant enhancement of the consolidation of declarative memories. 

So if you want some life advice from me - if you have a big test coming up, you might benefit more from a good night of sleep than cramming those extra few hours. 

One explanation for how this occurs is that when you’re awake - memories are encoded in the neocortex and more strongly in the hippocampus. During slow-wave sleep, sharp wave ripples are observed that accompany a memory replay of encoded information in the hippocampus which stimulates the transfer of memory-related information to the neocortex. There, these memory traces are strengthened and ready for recall so you can remember that WWII started September 1, 1939, for your next History Exam. 

So what about naps? Naps generally range between 20 and 90 minutes,  which is not really enough time to complete a full sleep cycle. 

So are there benefits to naps? And how do their lengths affect those benefits?

It has been generally proven that naps improve mood, reduce sleepiness, increase alertness, and yes! Improve memory consolidation. 

One experiment showing this was where people were invited to spend some time in the lab of Harvard sleep scientists Professor Robert Stickgold. They were trained to navigate their way through a virtual map at around lunchtime and then tucked up into bed shortly afterward. Scientists monitored their brain waves and woke them up shortly before they fell into REM sleep. They were able to show that the nappers were able to perform much better at navigating the maze than participants who didn’t nap. 

So these participants are probably making use of that non rem sleep to block interference from external stimuli and strengthen and consolidate those memories, making them more capable of navigating the mazes. We are also probably making up any sleep debt that we may have incurred during the course of the previous night’s sleep or lack thereof. 

Naps appear to be especially important for young children. I think that this is because when you’re young, you’re constantly being bombarded by new information and new experiences and your brain is rapidly developing to ensure that all of those memories are consolidated, naps, that extra sweet sweet slow-wave sleep is super important. 

Those benefits definitely extend to adult humans as well but perhaps are not as crucial as when you’re still wearing diapers. 

The second part of the question becomes - are there any downsides to napping? How could there be??? It’s one of the greatest things in the world!

But if you’ve ever taken one of those - get home from work, konk out, and don’t know what time it is when you wake up kind of naps, you know that you might feel groggy and unmotivated to do anything afterward. In addition, you might mess up your sleep schedule later in the evening, end up getting less sleep overnight, and suffering the consequences. 

But one study examined the correlation between total sleep, accounting for both nighttime sleep and naps during the day, and the risk of death and major cardiovascular events such as heart attacks. They found that if you are getting between 6-8 hours of sleep a night, you are at the lowest risk of death and cardiovascular events. In addition, daytime napping is associated with an increased risk of death in people with more than six hours of nighttime sleep but not those with less than six hours of sleep. So if you are napping to make up for lost nighttime sleep - no extra risk but if you're sleeping more than you're supposed to - your risk increases. 

I feel like I’m being a bit captain obvious here. I get the feeling that if you are sleeping much more than you should be and are taking more naps as a result - there’s probably something wrong with your health. Your body is fatigued and that might be a sign that you are not in good health and you might want to get checked out. 

I want to leave you with quote scientifically-backed unquote advice on how to take the perfect afternoon nap. You should eat a delicious, nutritious lunch, and then right around 1-3 PM you should find a cozy comfortable corner and set an alarm for 20-40 minutes. Push to 90 if you’re making up sleep debt. Too much longer and you might wake up groggy but a solid half-hour will get you the magic power of memory consolidation, leaving you ready to stuff your brain with more knowledge. 

But that is a bite-sized overview of the neuroscience of sleep, memory and ye good old afternoon nap. I want to highlight that this is an incredibly active area of research and new theories and research papers are coming out every single day. I’ve cited quite a few by Professor Matthew Walker over at UC Berkeley in CA so I recommend heading over his lab website and maybe reading his book - Why We Sleep. 

I hope that you enjoyed the episode and you learned something new! I’ve cited all my relevant sources and papers in the show notes and you should keep an eye out on Instagram for some cool figures I think are pertinent. 

Please rate, review, and subscribe and if you have any questions, comments, concerns, queries, or complaints please email me at neuroscienceamateurhour@gmail.com or DM me at NeuroscienceAmateurHour on Instagram. This podcast is available on pretty much any platform I can think of so please recommend it to your friends and loved ones! Also if you have something you really want to learn about - please contact me and you’ll probably see an episode about it soon!

Happy researching! Hope to see you again soon!