April 15th, 2019
Sleep has always been a necessary function of everyday life for all mammals. From an evolutionary perspective this cannot be a mistake as during the period of sleep you are vulnerable to predation. Therefore, there has to be a really good reason for mammals to sleep for extended hours. What is that reason? Why are toddlers and frankly all of us more cranky when we do not sleep enough?
Dr. Peter Attia and Dr. Matthew Walker's recent podcast on Drive was the impetus for this newsletter. They deeply dove into the deep end of the sleep pool and answered a host of questions
that have been swirling around in my mind since I purchased an OURA ring to measure sleep quantity and quality.
"Sleep Shrinks the Brain"
So says an article in the February 2017 edition of Scientific American.
Research by Dr. Cirelli in the Journal Science is shedding light on the way that we formulate memories while cleaning up our brain's storage from cluttered information. Dr. Cirelli and her team studied mice and the effects of sleep on the brain's circuit connections called synapses. They found that during sleep the brain shrinks as weak neuronal synapses are cleaned out or pruned. They hypothesize that by removing these synapses the brain allows for new memories to be made.
They looked at the brain slices of formerly sleeping and awake mice under high powered electron microscopy and noted that the synaptic connections shrank by 18% for 4 out of every 5 neurons. The larger and more stable neuronal synapses were left alone. This is likely because these neurons hold significant information that the brain has deemed necessary for preservation, i.e. serious memory for self preservation. I would think of a bull chasing you and you survive the event. That is a memory that you do not want pruned as it is advantageous to remember an event that is life threatening. In the study, all other neurons were paired down in size in a nightly ritual of restoration that allows for lots of new synaptic connections to be made for new memories.
This may be the first clear anatomic evidence that we are consolidating important memories while removing less important ones while we rest. This scientific discovery adds more fuel to the, "we need to sleep and recharge adequately" debate.
Dr. Walker states, "The silent sleep loss epidemic is one of the greatest public health challenges we face in the 21st century." According to the podcast, Dr. Walker notes that American adults are getting on average less than 7 hours of sleep nightly which has profound effects on many neurological and biological systems. He likens this challenge to be right up there with poor nutrition.
What happens at night while we sleep other than memory production and the culling of unneeded neurons? It turns out that the research team of Dr. Nedergaard at the University of Rochester found that the brain has a pseudo lymphatic drainage system that takes out the brain's sewage at night called the glymphatic system. (Lliff et. al. 2012) This turns out to be critical as the lymph systems of the body clear out unwanted proteins and junk that can cause local inflammation anywhere in the body. In Dr. Lliff's research they found that the beta amyloid protein associated with Alzheimer's disease was cleared out by this glymph drainage system. This is profound knowledge for at risk humans.
In the Journal JAMA Neurology, Dr. Ooms and colleagues looked at the amount of beta amyloid protein removed during sleep deprivation in humans over the course of one night. The results are not encouraging for people at risk for Alzheimer's dementia and also deprive themselves of sleep. The subjects that were sleep deprived had statistically significant decreases in amyloid protein clearing. (Ooms et. al. 2014) (Benedict et. al. 2015)
I think back to my medical school and residency years and wonder how the chronic sleep deprivation was effecting my memory consolidation and mood, not to mention the clearance glymphatic function. We would routinely work north of a hundred hours a week. At best that left me with 68 hours of sleep time per week which would have been fine if I did nothing else outside of the hospital. Clearly, anyone that knows me knows that this could not be true even if I had not met the love of my life intern year. Thus, I was working on limited sleep chronically.
Let us look at memory again. From Dr. Walker's 2008 Sleep Medicine Article: "Studies using procedural and declarative learning tasks have demonstrated the need for sleep after learning in the offline consolidation of new memories. Furthermore, these consolidation benefits appear to be mediated by an overnight neural reorganization of memory that may result in a more efficient storage of information, affording improved next-day recall. Sleep before learning also appears to be critical for brain functioning. Specifically, one night of sleep deprivation markedly impairs hippocampal function, imposing a deficit in the ability to commit new experiences to memory. Taken together, these observations are of particular ecologic importance from a professional and education perspective when considering that sleep time continues to decrease across all age ranges throughout industrialized nations." (Walker M. 2008)
In other words, when we obtain inadequate sleep volumes or quality, we are at risk for worsened memory consolidation. Think of this issue in the context of a teenager who has to rise up too early because archaic governmental rules prizing parents work hours over a child's learning. See this SPA newsletter for more on this topic. Teenagers by nature stay up later and are potentially studying far into the evening interrupting their sleep memory consolation time.
Let us pause here! So far we now know that humans at all ages will suffer from memory dysfunction and brain sewage cleanup problems leading to inflammation and damage long term. What ages are most at risk are likely mirrored by other physiologic events that are at risk based on age. Teenagers, infants and toddlers are rapidly growing creatures requiring more macro/micronutrients, water, and toxin avoidance for success. It is likely that sleep follows these same principles.
Part II next week:
Scientific American Article
Czeisler JAMA Pediatrics Article
UCBerkley News Article
Lliff Science Translational Medicine Article
Drive Walker Podcast
Ooms JAMA Neurology Article
Walker Sleep Medicine Article
Teens and Sleep Newsletter
Benedict Alzheimers and Dementia Article
Gottlieb BMC Medicine Article
Paiva Sleep Medicine Article
Chaput BMC Public Health Article