Sleep and Dreaming - Part 1 - Part 2
Sleep is another process in the brain that we still have much to learn. We know that while we sleep, we cycle between two very different states. The first is called slow wave sleep characterized by long waves of undulating electrical activity. The second is rapid eye movement (REM) sleep characterized by frantic brain activity that looks very much like wakefulness. It also has very obvious physical signs: the rapid flickering motion of the eyeballs, the near-total muscle paralysis (to prevent from acting out the dreams), and penile erections.
It is found that dreams are associated with REM sleep. During dreaming, the visual cortex is very active (to generate internal imagery), as are the amygdala, thalamus and the brainstem, which fits with the fact that dreams tend to be very visual and emotional. At the same time, the prefrontal and parietal cortices and the posterior cingulate, areas which deal with rational thought and attention, are all very quiet, which tallies with the lack of insight, illogicality and time distortion that characterizes dreams. Although the hippocampus is actively processing long-term memory, the short-term memory region is inactive, which explains why dreamer forgets what just happened.
It is now known that REM sleep falls into two types, generating two different kinds of dreams. Firstly there is the tonic component. It is accompanied by muscle relaxation and sometimes sexual arousal. Tonic REM takes place earlier on in the sleeping cycle. It is calmer, more restful, and more passive. When woken, the dreamer typically reports such things as "I was feeling floaty" or "there was a feeling of peace". The second type of REM sleep is known as phasic and is characterized by jerky eye movements, spasmodic limb and facial twitching and sudden breathing changes. When volunteers are woken from this sort of REM sleep, they typically describe their dreams as being strongly visual, active and "real". Phasic REM and its accompanying dreams tend to occur later on in the sleeping period. Nightmares are associated with this type of sleep.
REM sleep appears to have arisen quite early in evolution - reptiles, birds and mammals all do it. Therefore, it must serve a very useful function. There are many theories of sleep function, which fall into four broad classes: restoration and recovery, predator avoidance, energy conservation and information relocation from short-term to long-term memory, similar to the transfer of data from disk to tape in the IT (information technology) industry. But not one of them has been confirmed or refuted.
Recent research in 2003 indicates that non-REM sleep may give brain cells a chance to repair themselves, and REM sleep may allow the brain's neuron receptors to recover (regain full sensitivity). It is found that Animals born immature require more REM sleep. Thus REM sleep may also act as a substitute for the external stimulation that prompts neuronal development in creatures that are mature at birth. Sleep research will identify the brain regions that control REM and non-REM sleep. It will lead to a more comprehensive and satisfying understanding of sleep, its functions, the mechanisms and evolution. It will probably gain insights into exactly what is repaired and rested, why these processes are best done in sleep.
is now realized that sleep is an actively regulated process, not simply the passive result of diminished waking, and that sleep should be regarded as a reorganization of neuronal activity rather an cessation of activity. It is found that the vigorous brain activation of REM sleep occurred at regular 90-minute intervals and occupied up to 20% of sleep. Even during NREM sleep, when consciousness may be totally obliterated, the brain remains significantly active. There is only a 20% reduction in cerebral blood flow during sleep. Although consciousness is dulled, the brain is still roughly 80% activated and thus capable of robust and elaborate information processing. It is amusing that all these activities occur inside our brain every night but we have only a dim notion of what is going on.
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