The idea that sleep has a reparative effect on the brain is longstanding. Even outside neurology, such a claim seems obvious to anyone who’s ever experienced the bone-deep fatigue of prolonged wakefulness and the refreshment of quality sleep (that is, every vertebrate who ever lived).
But while there’s almost a century of evidence from psychology on things like long-term memory consolidation, we still don’t really know what the physiological mechanism is. As Dr. Anna Majewska, assistant professor, Department of Neurobiology and Anatomy, Center for Visual Science at the University of Rochester Medical Center and post doctoral fellow, Department of Brain and Cognitive Sciences at MIT says, “Sleep serves a million functions. Figuring all those out and how they intersect is very hard.”
We’ve seen research that seems to confirm our suspicions about sleep’s role in neural regeneration. A study published in Neuron called it the “price we pay for plasticity.” As the researchers wrote, the increase in plasticity that happens as we learn and experience inputs (expressed physically as connections between neurons) can’t be sustained for long because it consumes energy, cellular resources, and space. The more it happens, the less efficient and more erratic neural signaling gets, affecting learning and memory (we experience that as being simply tired).
The only answer is to put the brain into an “offline” state where it can consolidate the strength of connections without the constant flow of inputs that happens when you’re awake.
Now, two recent studies have emerged that seem to further cement what we all assume into scientific fact. The brain takes advantage of our sleeping hours to do a host of tidying and organizing, like an after-hours office cleaner getting the workspace ready for tomorrow.
The Self-Repair Principle
We already know, from research published in Science, that interstitial and cerebrospinal fluid clear amyloids — a starchy protein that left unchecked, can lead to Alzheimer’s disease, Parkinson’s, and other disorders — from the brain up to 60 percent more when we’re asleep.
Now Majewska and her team have found in research published in Nature Neuroscience that immune cells called microglia are also more active while we sleep, busily reorganizing connections, fighting infections, and repairing damage. “In the sleep state, microglia appear to be more dynamic and can scan the brain more efficiently to find locations that need their intervention,” the researchers say. “They are more able to respond to injury and therefore may be the most active cleaning up debris and repairing the brain. They also appear to be more able to interact with neurons, and synaptic structures in particular, suggesting that during sleep their roles in synapse remodeling and plasticity are the most pronounced,”
Another study, from the University College, London, Cell and Developmental Biology Lab, involved the quite amazing feat of “watching” the brain transport information into more permanent experience in the cortex from the traditional seat of memory (the hippocampus), filing memories and experiences away to other parts of the neural architecture to make them more permanent in the brain.
The process is far more pronounced when we’re (laboratory rats who’ve just run a maze, in this case) asleep. If you interrupt what lead researcher Dr. Freyja Ólafsdóttir calls the “reactivation” of memories in the brain, the animal learns a given task much more slowly, indicating that the actions required aren’t yet part of entrenched, learned experience.
The study showed that certain neurons that deal with spatial awareness and environment come alive while the rats navigated mazes and tracks thanks to electrodes that connect directly to brain areas critical to memory formation like the hippocampus. The pattern of cell activity becomes what Ólafsdóttir calls a neural representation of a particular experience.
She and her colleagues then saw those same patterns of cells firing while the rats subsequently slept not in the hippocampus where they first formed, but in the output cortical regions. It was the experience or memory not just being replayed, but being replayed somewhere else in the brain to be stored in long-term memory.
Seeing the brain process, transfer, and file a memory away in living color is exciting enough, but it opens up even more exciting questions for researchers. Ólafsdóttir’s next area of enquiry, for instance, will be how memory formation relates to infantile amnesia, the property where the brain isn’t really directly aware of its own existence and the experiences and memories it stores until several years into life.
It’s only when self-awareness comes online can the brain start its useful life as a memory tool, and as Ólafsdóttir says, that point rather than birth might mark the true onset of brain maturation.