Why Do We Sleep?
neuroskeptic.blogspot.com
neuroskeptic.blogspot.com
One of the functions of sleep might be just inactivity but another much more important one must be associated with neurons.
I think sleep (namely REM phase) is needed for neural network to avoid overlearning. That's why sleep deprived persons hallucinate. Their neural network is overlearned to recognize stimuli from senses and it recognizes known patterns even if stimuli is week. Nothing better to avoid that is to disconnect neural network from muscles (to avoid hurting yourself) and discharge neurons randomly for some time.
The animals that "never" sleep are actually just failing-over to the other processor while the GC happens.
If Siegel's theory were true than people wouldn't die from lack of sleep. At least, not if they were given enough energy to sustain themselves without going dormant. But that doesn't work, people do die from lack of sleep. So how can someone make the argument that sleep isn't serving some vital purpose?
Basic evolutionary theory would point out that even if sleep started out as a totally optional thing that could be skipped with no loss, that evolution would rapidly start using the sleep period as a hook to load in other things that provided an adaptive advantage. For instance, as long as you're going to be sitting there doing little-to-nothing in a controlled, predictable manner, why not load all your muscle repair into that time period, leaving your wakefulness period with muscles not concerned about repair? Why not load various cognitive batch processes into the sleep period, as long as it is there, leaving you with a brain that isn't worried about those things during the wakefulness period and therefore spending more energy on useful computation, or even just plain spending less?
I've actually directly observed this process in my evolutionary computation homework assignments. I choose that phrasing to indicate my not-extensive experience on the topic, and that even so, I've seen this, where chunks of useful code get loaded up with other stuff that bears some sort of relationship to the original. (And evolution has a strange idea of "relationship".) (I do sometimes think that evolutionary computation should be a required course for biologist PhDs; the hands-on experience is invaluable.)
I often wonder at "either/or" arguments in the context of evolution; to me it betrays a profound misunderstanding about the process. It is the very incarnation of a "both/and" process. Is sleep an optional resting period just to conserve energy, or a time for healing, recovery, and cognitive batch processes? Perhaps there was a brief time when the answer was one or the other, but today the answer is simply "Yes", in general. (Your Species' Mileage May Vary, I'm speaking to the general case.)
It seems for me that many of those who accept evolution, still thinks in creationist terms: that however the human body works today, must have been the original purpose. And that we are not the result of an evolutionary history filled with random input and short-sighted adjustments, thus ending up with something that could never have been conceived in the beginning.
Sometimes stuff "just came into existance" without an (initial) purpose. Purpose is not a pre-condition for existence.
Are there many species that sleep with their eyes and ears open?