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Why sleep?
There has been a lot of exciting new research in this relatively young discipline over the last 10 years but first let’s address a fundamental question: what is sleep for? An interesting approach is via the bio-sphere. We see extraordinary variability. Not surprisingly unicellular and simple multicellular organisms may not exhibit sleep equivalence but flies, for example, die of sleep deprivation. Rats do also. Birds, on the other hand may go weeks without sleep per se during their migrations. Dolphins allow one side of their brain to be in a state of deep sleep while they swim; some seals, go the whole hog and actually have contralateral paddle paralysis during their one-sided brain sleep! Apparently, the scientific jury is out whether frogs sleep! Hibernators, such as some bears, hedgehogs, bats (who only spend a small percentage of their lives awake) and marmots may spend months unarousable- though hibernation is not strictly sleep. All this to illustrate some of the huge differences in the use of what sleep researchers have thus termed only ‘adaptive inactivity’. In other words, there are few generalities and sleep has been adapted to that advantage of each organism.
As humans, we have the most sophisticated brains with 20% of our metabolism being spent on its maintenance and upkeep so not surprisingly we have our own sleep biological mechanisms. Of course, through most of our nasty, brutish and short lives in earlier history we were not able to lie abed- thus, sleep would have been taken in snatches or where safety was perceived for a while.
Apart from the obvious advantage in reduced energy expenditure what else is the human and particularly the brain achieving in sleep? We can determine from surface electrodes the different levels of sleep. Normal healthy sleep comprises an initial stage one where the trace is not markedly different from the awake state, going through to stage 2 where noticeably there are the onset of K complexes and sleep spindles (these are characteristic shaped movements in the electrical trace). In deep sleep the rhythm is more rolling and slow called delta wave sleep. These non- REM sleeps precede and interweave with periods of REM sleep where there is atonia (body paralysis- though fortunately not the diaphragm!- preventing us acting out dreams) during which the rhythm returns to a more waking state form. Recently, it has been proposed that memory consolidation may be reflected in these patterns- for example, the K complexes allowing memory to replay from the hippocampus (deep in the brain) to the neo-cortex (the frontal lobes); while, in deep sleep, the delta waves reflect downscaling in the hippocampus- effectively cleaning the slate for new memory acquisition. Sleep spindles are still a mystery but they may well represent local neuronal plasticity (where the brain cells undergo modification) in the neo-cortex.
