Sleep has been shown to help transform fresh, fragile short-term memories into long-term ones. Is it possible, though, that sleep's memory-boosting effect isn't limited to the brain?
That's the hypothesis from a team of researchers, published today in an opinion article in the journal Trends in Neurosciences, who believe that sleep also plays an essential role in the immune system's memory formation process, much in the same way as it does in the central nervous system.
The implication is that sleep could be a factor in how well the human body can fend off illness.
If this sounds far-fetched, keep in mind that biological memory, as the researchers define it, is simply the process by which the body extracts and maintains "relevant environmental information to enable sustainable adaptive responses." By this definition, it's not illogical that the way our brains form and store long-term memories and our immune systems form and file away antigenic information would share a similar methodology.
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"In both the central nervous system and the immune system, our capacity to store information is limited — we can’t file away every encounter," explains Jan Born, director of medical psychology and behavioral neurobiology at the University of Tuebingen in Germany and the article's lead author.
In response, both systems enact a system the researchers call "gisting," by which specific memories (in the central nervous system) and pathogenic information (in the immune system) are consolidated into abstract categories.
Let's start with the immune system: When a virus or bacteria enters the body, it is engulfed by an antigen-presenting cell that stores the specific antigenic information. For the body to develop a long-term defense against the pathogen, however, the information must then be conveyed to T cells (located in the lymph nodes). This is where "gisting" comes into play. Instead of copying the information verbatim, the T cells instead remove a condensed version with "much less information than the original pathogen — it’s an abstraction process," Born says. Not only does "gisting" save the immune system from storing excess antigenic information, but the abstractions allow T cells to recognize and attack pathogens that are similar, but not identical to, previously encountered viruses or bacteria.
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According to Born, a similar process takes place in the central nervous system, which is not equipped to store all the memories we develop each day. Instead, to transition from short-term to a long-term, memories are condensed and abstracted, says Born, who offers the following example: Say you just learned that Berlin is the capital of Germany. At first, your hippocampus would retain the memory's contextual details – you were in geography class on a bright sunny afternoon in early spring – but as the piece of knowledge is converted from the hippocampus to the neocortex, this context falls away. Says Born, "In the end what remains is just the abstracted schema-like version of that memory: that Berlin is the capital of Germany."
As with T cells the immune system, this also allows the central nervous system to easily categorize similar, but not identical, items, facts or events. (This is why we can easily categorize a house that we've never seen before as a house – even if the specific details are unfamiliar, we recognize variations of standard 'house' features and connect the dots.)
In the nervous system, slow-wave sleep (deep sleep, or not rapid eye movement sleep) has been shown to play a crucial role in the "gisting" process. And, if the researchers' theory holds, the same is true for the immune system. Born points to previous research, which has shown that vaccines are more effective when participants get enough sleep the night afterwards – an indication that sleep enhances T cells' ability to decipher, condense and abstract antigenic information. Just as it does to the brain, deep sleep boosts the immune system’s ability to “remember,” so to speak.
For now, Born stipulates, this is just a rather elegant theory. But he's pretty confident that additional research will back it up. Go ahead and tentatively add "immune system memory" to a long and growing list of all the reasons you should be getting enough sleep at night.
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