After a long day of hard work, there is nothing quite so welcoming as a cozy bed. Sleep is essential to how we function. Everything feels more challenging, tasks take longer, and everyone seems more annoying when we haven’t had enough sleep. For people who have gone days with little to no rest, the world becomes hazy and the mind distorted. Getting enough sleep even helps wounds heal faster.
We are still not entirely sure how far the benefits of sleep go or why we need so much of it. However, a team of neuroscientists at the University of Geneva (UNIGE) in Switzerland discovered that a good night’s sleep is essential to building and storing our memories. The team, led by Virginie Sterpenich, published a paper on their findings in Nature Communications.
Building Memories
When we sleep, many things happen—our heart rate slows down, our body temperature drops, and our brain cycles through various waves. We start with alpha waves, which are responsible for our consciousness. Next, we move into theta waves, a slightly unconscious boundary between being awake and asleep. (Theta waves also appear in meditation and deep relaxation.) Finally, we move into the deep, dreamless delta waves, where the brain builds memories.
During our dreamless delta wave stage—that is, non-rapid eye movement (NREM) sleep—our brains transfer what we have recently learned or experienced from our short-term to our long-term memory. In addition, the same region of our brain involved in our waking life is reactivated during NREM to enhance memory consolidation.
According to Matthew Walker, the author of “Why We Sleep,” when we enter this NREM stage, we “manage the finite storage space within the brain,” and our brains determine which “memories are fresh and salient, and which memories that currently exist are overlapping, redundant, or simply no longer relevant.” In a nutshell, our brains decide what it is that we should remember. This is also why it’s a good idea to get a good night’s sleep before an exam. It is most likely better for you than an all-night study session.
Positive Memories
The UNIGE team decided to determine how and why the brain selects memories to reprocess during sleep. Their first observation was that memories are more likely to be stored if they potentially aid an individual’s survival. Therefore, the brain stores those experiences that have a more significant positive or negative emotional association—known as “effective relevance”—because it will help guide our future behavior.
For example, I am more likely to remember a painful cut than a red bicycle passing by because the former is much more helpful to my future life—that is, do not direct a knife toward you while cutting fruit. Mainly, our brains prefer to store positive and rewarding experiences. The UNIGE team had participants play two games, some of which they won. The results showed that the volunteers remembered the details of the games that gave them a rewarding experience of victory.
When we have a pleasurable experience, the parts of our brain responsible for the feeling of reward (the hippocampus and the ventral-temporal-limbic system) are reactivated during our sleep, and our brains store these experiences as long-term memories. In brief, if something feels good, we are more likely to remember it.
These findings have potentially valuable implications for learning and memorizing information. For example, if we want to remember something, we are more likely to do so if we associate it with a positive experience. Therefore, if you plan to teach someone something, try giving them a prize when they get the correct answer.
Negative Memories
While the study suggests that positive experiences are more likely to be stored during our deep NREM stage of sleep, it leaves us to wonder about the role of sleep in our negative memories. A pile of evidence suggests that our negative experiences are reactivated during the REM sleep stage. A new wearable ultrasound patch that enhances REM sleep is now being tested as a noninvasive way to improve that stage. That means we are more likely to be storing negative experiences as memories when we are dreaming (which almost exclusively occurs during the REM sleep cycle).
Regardless, the UNIGE study did not have the dataset to prove this because their participants did not enter REM stages long enough to measure it. However, the findings suggest that when you have a night of prolonged and intense dreaming, you might be putting to memory those disappointing, sad, scary, or stressful moments of your day.
While sleep is very beneficial, it is essential to note that too much or too little sleep is unhealthy and can lead to cognitive decline.
Cognitive Decline
Like everything in life, sleep is best in moderation. However, a multi-year study of older adults found that short and long sleepers experienced more significant cognitive decline than people who slept moderate levels, even when the influence of early Alzheimer’s disease was considered. The study, published on Oct. 20, 2021, in the journal Brain, was led by researchers at St. Louis’s Washington University School of Medicine.
Alzheimer’s disease and poor sleep are associated with cognitive decline, and unraveling the effects of each has proven quite a challenge. To untangle the complicated relationship between sleep, Alzheimer’s, and cognitive function, the researchers generated crucial data by tracking cognitive function in a group of older adults over several years and analyzed it against Alzheimer’s-related protein levels and measures of brain activity during sleep. Their findings could aid efforts to keep people’s minds sharp as they age.
Brendan Lucey, MD, first author, and an Assoc. Prof. of neurology and director of the Washington University Sleep Medicine Center said, “It’s been challenging to determine how sleep and different stages of Alzheimer’s disease are related, but that’s what you need to know to start designing interventions. Our study suggests that there is a middle range, or ‘sweet spot,’ for total sleep time where cognitive performance was stable over time. However, short and long sleep times were associated with worse cognitive performance, perhaps due to insufficient or poor sleep quality. An unanswered question is if we can intervene to improve sleep, such as increasing sleep time for short sleepers by an hour or so, would that have a positive effect on their cognitive performance so they no longer declining? We need more longitudinal data to answer this question.”
Poor Sleep
Alzheimer’s is one of the main causes of cognitive decline in older adults, contributing to roughly 70% of dementia cases. Poor sleep is a typical Alzheimer’s symptom, and it can accelerate the disease’s progression. What’s more, studies have shown that self-reported short and long sleepers are more likely to perform poorly on cognitive tests. However, such sleep studies usually do not include assessments of Alzheimer’s disease.
To understand the different effects of sleep and Alzheimer’s disease on cognition, the team turned to volunteers who participate in Alzheimer’s studies via the university’s Charles F. and Joanne Knight Alzheimer Disease Research Center. Such participants undergo annual clinical and cognitive assessments and provide a blood sample to be tested for APOE4 (the high-risk Alzheimer’s genetic variant.)
However, the volunteers also provided cerebrospinal fluid samples for this study to measure Alzheimer’s protein levels. In addition, each slept with a mini electroencephalogram (EEG) monitor attached to their foreheads for four to six nights to measure brain activity during sleep.
Sleep Quality Over Sleep Quantity
The team found a U-shaped relationship between sleep and cognitive decline. Overall, cognitive scores declined for those that slept less than 4.5 or more than 6.5 hours per night (as measured by EEG), while scores remained stable for those in the middle of the range. According to Lucy, EEG typically yields estimates of sleep time an hour shorter and longer than self-reported sleep time; therefore, the findings correspond to 5.5 to 7.5 hours of self-reported sleep.
David Holtzman, MD, co-senior author and a professor of neurology, concluded, “It was particularly interesting to see that not only those with short amounts of sleep but also those with extended amounts of sleep had a more cognitive decline. It suggests that sleep quality may be key, as opposed to simply total sleep.”
Sleep is not a one-size-fits-all. Each person’s sleep needs are unique. Therefore, people who wake up feeling rested on short or long sleep schedules shouldn’t change their habits. However, those getting poor sleep should be aware that most sleep problems can be resolved.
