What it is:
Cognitive health depends upon a myriad of factors to keep in top form. We've narrowed the scope of lifestyle choices to fall under three categories: nutrition, exercise, and sleep. These stances provide the most recent research and actionable items to maintain a high quality of life.
Sleep offers a nightly opportunity for the brain to hit reset. Even the most restful days are biologically taxing, and sleep provides a precious window to rejuvenate and refresh us for what lies ahead.
Our nervous system is comprised of billions of neurons working together to allow us to think, learn, move, rest, and repair. This system allows us to interact with the world, work, play, and form memories.
Sleep is essential for neuronal function as it gives neurons time to rest. It also supports the health of specialized glial cells that allow neurons to perform their jobs. Sleep is generally considered the price we pay for our ability to learn, and also to forget. Without it, the system becomes toxic and malfunctions.
Purported claims:
Sleep is essential for:
Memory formation
Optimal cognitive function
The health of neuroglia
Proper metabolic waste clearance via the glymphatic system
Synaptic downscaling
What the science says:
Neuron Health
The nervous system consists of neurons and specialized support cells that act as the glue that holds the neurons together, collectively termed “neuroglia.” These neuroglia serve many vital functions, such as maintaining the blood-brain barrier, scavenging debris, and supporting the neurons to fire more rapidly.
Neurons use chemical messengers (neurotransmitters such as dopamine, serotonin, and glutamate) to communicate across the tiny spaces between them (known as synapses), sending signals that either excite or inhibit them. How these neurons form different connections changes all the time, and this is known as plasticity. The more that neurons fire together, the more they wire together.
While many different kinds of signals are happening between these neurons throughout the day, in general, our wake state is associated with excitatory neurotransmission. A natural consequence of all this neuronal firing is the production of metabolic waste products, including reactive oxygen species (free radicals), which can damage cells and - if left unchecked - contribute to cell death. Sleep - especially non-rapid eye movement (NREM) sleep - is mainly associated with inhibitory neurotransmission, thus providing some downtime for the neurons to rest. During sleep, melatonin, a potent antioxidant, circulates the brain, scavenging these free radicals before they can do significant damage. Sleep also allows our cells to repair misfolded proteins, replenish neurotransmitter stores, recycle materials, and allow the energy-producing mitochondria to rest.
The Glymphatic System
While melatonin does its job, and the neurons rest and repair during sleep, the brain also switches on a pump system that serves to flush out these toxic metabolic waste products en masse. While this pump is somewhat active around the clock, it wasn’t until 2013 when a groundbreaking study led by neuroscientist Maiken Nedergaard, showed just how important sleep is for this process to keep up with the demands of waking life.
The pump circulates cerebrospinal fluid (CSF), which is a clear liquid that surrounds the brain and spinal cord. CSF contains oxygen, glucose, white blood cells, proteins, and essential ions such as sodium and potassium. Adults have about 80-150 ml (3-5 oz) bathing their brain and spinal cord at any one time, and it is continuously made and flushed out, at a rate of about 400 ml (13 oz) per day.
CSF acts as a cushion so that the brain and spinal cord safely float beneath the skull and spine. This liquid also offers immunological protection for these vital organs and creates an environment that optimizes neuronal firing.
CSF is made when specific blood components (mentioned above) are forced from the bloodstream through specialized cells into small, hollow chambers within the brain. The liquid then flows around the brain and down the spinal cord, taking with it any waste products.
During sleep, this process ramps up. Some of the cells in the brain shrink, which increases the size of the space through which this plumbing system can get to work and clear out toxic proteins like amyloid beta (Aβ), tau, and alpha-synuclein (αS), which - if allowed to collect - cause inflammation and eventually cell death. This is known to cause neurodegenerative diseases such as Parkinson’s and Alzheimer’s.
While these diseases have a genetic component, chronic sleep deprivation is a significant risk factor. Slow delta waves that occur during deep sleep can increase the space available for CSF flow by up to 60%. And sleep spindles, which are rapid bursts of neuronal firing that occur during stage 2 (N2) sleep, appear also to promote waste clearance. Glymphatic activation also occurs during anesthesia.
Glymphatic flow is generally limited during the day and becomes even more restricted with prolonged wakefulness, such as after a night of sleep deprivation. Norepinephrine (also called noradrenaline) is both a hormone and neurotransmitter that is involved in the body’s sympathetic nervous system, otherwise known as the “fight or flight” response. It has been shown to inhibit glymphatic drainage and increases during sleep deprivation.
Interestingly, there also appears to be a circadian rhythm of the glymphatic system, as shown in experiments where mice are exposed to constant light for several days. Glymphatic flow may not be as effective during the night if the person (or animal) is awake and missing out on those delta waves and sleep spindles, but there may still be some waste clearance happening at night. Importantly, this highlights the risks associated with circadian misalignment, which is when the body clock is desynchronized from the environment and can come from frequent travel or exposure to too much bright light at night.
Synaptic Downscaling
As well as flushing out toxic waste, sleep is widely accepted as the price we pay for our ability to take in new information each day. The morning after a good night’s rest, we are refreshed and able to focus. But the longer we are awake, the closer our brains get to the point of saturation. Imagine spending an entire day proofreading the same report; there comes a point where the information stops being absorbed.
What’s happening at a cellular level can be best visualized with the analogy of a forest fire. Lands prone to wildfires typically consist of mature trees surrounded by highly flammable grasses or bushes covering the forest floor between them. With regular, minor, controlled burns, this low-lying kindling is removed, leaving the trees relatively isolated and protected from the spread of fire. But if these minor burns do not happen, the flick of a cigarette can send the whole forest up in flames.
In this analogy, the trees are the neurons, and sleep is the small, controlled burns of the kindling. The longer we are awake, the more crosstalk there is between different neurons. We need sleep in order to sift through what is relevant crosstalk and what is useless information.
Our survival depends upon the brain being primed to recognize environmental anomalies. It used to help us distinguish when to run from danger, but we still need its sifting capabilities in our modern lives. The longer we are awake, the less sharp the brain becomes at detecting these anomalies because of all the other information that it is simultaneously storing: the weather forecast, what we ordered for lunch, or the phone number for the electrician. The temporary storage of this information is necessary, but it does require biological resources. Sleep sifts out the items that are safe to forget, while ensuring that we retain the essential thoughts.
During sleep, neurons can grow or shrink their branches to enforce important connections (such as learning a new skill) or delete irrelevant ones (such as the face of the delivery man that we may never see again.) While we sleep, there are continuously changing patterns of spontaneous brain activity that allow neurons to obtain a big picture of the brain’s overall knowledge of the environment - acquired over years of evolution and development. During moments of wake, we experience only a limited snapshot, and - depending on the day - this may be heavily biased. For example, a day at a conference or on vacation probably isn’t considered a “typical” day. Sleep is essential for placing the experiences we have each day into a greater context depending on the experiences we have already had in our lives. This is known as synaptic downscaling and is crucial for peak daytime performance.
Cognition
The memories we retain largely depend on what synapses already exist in our brains. That is why learning takes time and is biased toward what we have already experienced. But consistent sleep supports the plasticity of our brains to adopt new skills and thought processes with time and dedication. In other words, condition yourself with a new behavior day after day, and eventually, it will become ingrained in your neural circuitry.
Memories are initially stored in a part of the brain called the hippocampus. During sleep, they are transferred to the cortex for long-term storage. Slow delta waves and the sleep spindles that occur during stage 2 (N2) sleep are important for this transfer. On the other hand, REM sleep plays an essential role in forming emotion-based memories.
While some types of memories (such as motor skills) can form during wake, many can only occur during sleep. Therefore, consistent rest is essential for optimal learning and daytime function.
Many studies have shown that people are better at recalling pictures and words after a night of sleep, but there are many other ways that sleep supports cognition. After 24 hours of wake, psychomotor performance is equivalent to having a blood alcohol concentration of 0.1%, which is over the legal limit in most states. This means that reaction time is severely impaired, with significant implications for health, safety, and general performance. Studies have also shown that we become more impulsive, and feel more emotional in a sleep-deprived state. Combined, these deficits prevent us from showing up as the best version of ourselves and could mean the difference between securing the deal, contributing the winning idea, and solving the problem the quickest or not.
Our take:
Sleep is essential for brain health and that of the nervous system more broadly. While there is no avoiding genetic predispositions and life’s many ups and downs, there are many ways to promote longevity and peak performance before any signs of cognitive decline emerge.
Prioritize a regular sleep schedule as best you can. It will keep you sharp and able to focus during the day, as well as enhance your capacity to learn new skills and retain information. Know when to draw the line at the end of a busy day and begin a wind-down process that works for you.
Consider the importance of maintaining a healthy circadian rhythm or body clock that is in sync with its environment. The easiest way to do this is to get adequate exposure to bright light in the morning and keep the lights dim at night. Frequent travel disrupts this rhythm, and while there are preventative and recovery measures for getting back on track as quickly as possible, this may impair glymphatic function, given what the science shows about its circadian rhythmicity.
If you are struggling to sleep, seek help. If you suspect you might have sleep apnea talk to your doctor. And if you are struggling with insomnia, consider a behavioral sleep medicine specialist. Remember that while your sleep tracker is useful, it may not show accurate data. The technology simply isn’t there yet. Don’t panic if you aren’t getting as much deep sleep as you think is ideal — we experience a natural decline as we age, and there is good evidence showing that the lighter sleep stages offer powerful benefits too.
Will this benefit me?
Although we need to pay attention to sleep, health is multidimensional and requires multiple areas of focus, including nutrition, physical activity, and stress levels. Take small steps to improve each of these factors, and you will notice improvements in areas you didn’t anticipate. For example, it has been shown that those who exercise tend to get better, deeper sleep, as do those who eat an anti-inflammatory, high-fiber diet rich in omega-3 polyunsaturated fatty acids. Focus on small changes for big results.
Still curious to try it? Here’s what to look out for:
As robust data continue to emerge about the many ways that sleep is essential for total mind and body health, it can be easy to put pressure on yourself to perfect it. But try to remind yourself that it does best when left alone.
Also, remember that you can do many other things alongside sleep to get added benefits. Reduce your alcohol consumption and limit caffeine intake to the first part of the day. Use stress-busting tools like yoga and meditation too. Quiet wakefulness has been shown to improve memory formation and decrease the production of the toxic proteins that lead to neurocognitive decline. How can you promote more calm in your life today?
References:
Sleep and the price of plasticity - https://pubmed.ncbi.nlm.nih.gov/24411729/
Discovery of the glymphatic system - https://pubmed.ncbi.nlm.nih.gov/24136970/
A beginner’s guide to the glymphatic system - https://pubmed.ncbi.nlm.nih.gov/25947369/
Glymphatic system - https://pubmed.ncbi.nlm.nih.gov/34699796/
Glymphatic and lifestyle choices - https://pubmed.ncbi.nlm.nih.gov/33212927/
Circadian control of glymphatic system - https://pubmed.ncbi.nlm.nih.gov/32879313/
Sleep and Parkinson’s - https://pubmed.ncbi.nlm.nih.gov/31227656/
Blood brain barrier and sleep - https://pubmed.ncbi.nlm.nih.gov/31253251/
Sleep and pain - https://pubmed.ncbi.nlm.nih.gov/26498229/
Sleep and memory - https://pubmed.ncbi.nlm.nih.gov/31451802/
Sleep and memory - https://pubmed.ncbi.nlm.nih.gov/23589831/
Yoga and neuroprotection - https://pubmed.ncbi.nlm.nih.gov/26029093/
Meditation and cognitive decline - https://pubmed.ncbi.nlm.nih.gov/24478663/
Neuroprotective effects of exercise - https://pubmed.ncbi.nlm.nih.gov/30564545/
Memory and sleep -https://pubmed.ncbi.nlm.nih.gov/23354387/
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