I have recently become interested in sleep: why we do it, what purpose it serves. So what follows are possibly incomprehensible musings. I make no guarantee they will make sense :-)
There are lots of theories about why we sleep, but none of them really explain it fully IMO. One theory is that it is required for mental health. But non-human animals, which probably do not have the same psyche, intellect, extent of experiences (and thus requirement for memory) as humans, still sleep. Even my two pet rats, whose experience each day is pretty much identical and presumably do not have any requirement to re-organise thought patterns or consolidate memories or work through subconscious issues, sleep a lot.
So there must be some physical reason for it. If it were just to rest and conserve energy then we could just keep still. Chemical energy fuels the cells of our bodies, and if we wanted to conserve that energy we could just stop moving. Of course our bodies are using energy all the time even when we're not moving, but the same applies whether we are sleeping or just resting.
If instead of sleeping we just stayed still, our bodies would have a chance to rest and conserve energy without needing to enter an altered state of consciousness. But this is not what happens. Maybe when we become unconscious our bodies rest even more when we are just motionless because of physiological changes which occur during sleep (i.e. slowing of heart rate etc). So perhaps the unconsciousness is a consequence of those changes - in other words, our bodies need to rest thoroughly, and we are unable to maintain consciousness in that condition.
However, none of that explains the neurological changes which occur during sleep. Brain wave patterns alter dramatically, and we even have unconscious experiences which cross in to our conscious thoughts (e.g. dreams which we can remember). But why? As I said previously, even animals which have little variety in experience and no apparent need to re-organise or resolve their thoughts enter a sleep state. Perhaps the brain-wave changes and experiences of dreaming are actually side-effects of the changes in consciousness, which are in turn side-effects of physiological changes, rather than being the actual purpose of sleep.
This has made me think about tiredness. There is a very real difference between feeling physically tired, for example after walking a long distance and your legs feel tired, and feeling tired in the sense that you are aware that you need to sleep. In the former case, the feeling of tiredness is very physical: your legs feel exhausted, heavy and weak, and you feel like you need to rest them. In the case of feeling that you need to sleep, the sensation of tiredness is difficult to localise. Where are you sensing that you are tired? You don't necessarily feel physically exhausted just because you need to sleep. They often occur simultaneously, but the feeling of needing to sleep is still qualitatively different to the feeling of needing to rest. Which makes me believe that resting and sleeping are qualitatively different themselves. But why?
I wonder whether anyone who started reading this is still awake? :lol:
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moto
