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I like starting our meeting “What is sleep?” with a session on function, because sleep looks different in different animals. so to figure out what sleep is we need to figure out what it does, then figure out what is necessary (conditions) to do that function then look for those conditions across ages and species.
Of course we have to start with some fundamentals and commonalities of sleep, even though we don’t know if they do - or should - define sleep or whether it is incidental-- a side effect, instead of an essential to sleep. I believe that thanks in part to the people here today, we have that starting point. We also have some basic functions of sleep identified, although it has been messy because until recently we have not known enough about those functions to adequately test whether sleep is important to those functions. But, again, we are at the point where we we have enough clues to start our reverse engineering phase.
In those early days we could not reverse engineer because we haven’t known what brain areas were responsible for what kind of learning and what neurotransmitters and electrical activity signatures were needed. But now, thanks to a horde of thousands, we know a few essential things:
Potentiation/depotentiation, engrams, ARC, cFos, mRNA, protein synthesis,
Circuits: what is interconnected. How specifically NE and ACh targets forebrain,
Order of play: cortical registration, Hippocampcampal assembly, and eventual cortical strengthening...
Now we are ready to roll up our sleeves and figure out what sleep really is!
Why awakening-based sleep deprivation is not great:
# As Jerry Seigel points out, if you wait until an animal shows electrographic or behavioral sleep beforee you awaken it, then you get the arousal-response.
# My 2016 Sleep paper shows (replicated by Duran et al., 2017) that cortical electrographic signs of sleep misses much subcortical sleep even normally... how much more when sleep pressure is high! We also show that hippocampus is in REM when when the neocortex remains in NREM normally, but even more when the hippocampal homeostatic drive (learning-induced) is high. Thus, any sleep deprivation protocol that relies on posture, EEG, or muscle tone would and probably does miss a lot of sleep that goes on "locally". Thus any negative finding, e.g., "REM sleep is not important for X learning task" could be completely wrong if we are not measuring that particular type of sleep from the area involved in that type of learning and/or memory consolidation. Recommendation: measure the amount of sleep from the actual structure essential for the function you are testing.
# Some of the sleep deprivation studies use pharmaceuticals like caffeine or some other stimulant, which is good because it eliminates the awakening response discussed in point 1 above, but of course stimulants have their own effect on the function under study.
----------On time variances of total sleep time vs sleep bout lengths----------
One function of sleep, like the memory process of sleep, may take only a few minutes or even seconds to accomplish, while other functions of sleep, like repair and clearance, may take much longer to accomplish, and be counterproductive to the learning function, and therefore sleep may need to intersperse the learning function at intervals during the longer sleep period in order not to lose, e.g., what you consolidated.
Either the undoing of one sleep state's function by another or the processes or side effect outcomes of one state requiring another sleep state to clean up after it may be why healthy sleep proceeds in an orderly fashion and unhealthy sleep, even if it is adequate in total length is disordered: REM sleep occurring too soon in depression, REM sleep occurring at sleep onset in narcolepsy, etc. If REM sleep serves to clean up synapses (strengthens those spuriously weakened but tagged for keeping, and weakens synapses spuriously strengthened by SWS or those ready to be erased now that slow-oscillation coupled spindles have consolidated them to their final place) then that would explain why the length of REM sleep is related more to the length of the prior NREM state than to the length of waking.
It may be that the cycles of sleep are timed as they and the states follow each other in order as they do because should our planned long sleep cycle be interrupted, we will at least get some of all of the different kinds of necessary work done. even though having only one cycle leaves it incomplete, it would be better than nothing. Interrupting a sleep cycle midway (e.g. being awakened out of SWS) may be like interrupting the wash cycle midway, leaving clothes soaked and soapy rather than rinsed and clean. A power nap with only stage 2 sleep may be like a quick rinse and spin-dry which is better than the dry but filthy state that the party of wakefulness has left us. But a sleep disturbed from slow waves may, like the middle of the wash phase with soap and soaking wet, leave our brains in a worse state than the previous dry filthy one.