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SleepingThere are key mental states each characterised by its own brain wave pattern <5>. When awake we can be attending or concentrating, or we can be relaxed. When asleep we could be in SHALLOW sleep, DEEP sleep, or REM sleep. Shallow sleep is often referred to as `Stage 2` sleep, and Deep sleep as `Stage 4` sleep. During REM sleep (Rapid-Eye-Movement sleep), the eyes move rapidly and continuously. At times REM sleep is referred to as dreaming sleep and sometimes called paradoxical sleep, or called paradoxical sleep only when referring to animals. During Deep sleep the body`s muscles are relaxed, heart beat and breathing are slow and regular. In REM sleep the body`s muscles are paralysed while heart beat and breathing fluctuate as they would during emotional upsets in waking life. Brain-wave frequency of the different sleep stages we pass through in the course of a night are outlined in Figure 1 `Sleep Pattern: Day-Night-Day`. From being wide awake before going to sleep, we relax, sleep lightly (shallow) for ten to fifteen minutes before sleeping deeply. Following Deep sleep we REM sleep after which we wake up through relaxing to being fully awake. <6> Figure 1 Sleep Pattern: Day - Night - Day Back to Contents list Deep sleep is followed by REM sleep. In order to achieve this as far as possible within a night, the brain arranges alternating periods of deep sleep followed by REM sleep <8>. The illustration shows graphically what happens to the brain`s electrical activity as the night progresses, illustrated by the frequency of the brain waves. As we progress from being awake through sleep to being awake again, the frequency drops, reaching its lowest point while in deep sleep and then rises again to the wide-awake level. Amplitude, that is voltage, changes inversely. It increases when the frequency drops, reaching its highest level during deep sleep, and then decreases again to the wide-awake level. Considering adults, that is excluding the young and the elderly, on the whole we `Deep Sleep` during the first half of the night, and `REM Sleep` during the second. But possibly because we cannot be certain how long we will sleep, whether our sleeping period will be interrupted unexpectedly, Deep sleep and REM sleep are divided into shorter sleep periods which alternate, something like: One Sleep Period (One complete night)One Sleep Period (One night) Deep Sleep (minutes) REM Sleep (minutes) Beginning of Sleep Period 40 7 20 13 25 10 25 End of Sleep Period Figure 2 One Sleep Period (One Night) Back to Contents list That we Deep sleep first and that REM sleep follows Deep sleep is clearly shown in Figure 2. This shows how the two kinds of sleep alternate as Deep sleep ends and REM sleep begins and proceeds. Deep Sleep and REM Sleep each take up about 20 to 25 percent of the night`s sleep. The remainder is largely taken up by transition `Shallow` sleep` periods which enable brain and body to adjust to the next type of sleep, and by occasional brief periods of intermediate `Stage 1` and `Stage 3` sleep <6>. DEEP SLEEP AND REM SLEEPWe have already seen much about Deep sleep and about REM sleep so this seems a good point to include in this section also what has been said so far. Both Deep sleep and REM sleep appeared about 180 million to 130 million years ago in mammals as they evolved from reptiles. Deep sleep and REM sleep are the core sleep activities, each taking up about 20 to 25 percent of the night`s sleep, the remainder being taken up by shallow transition sleep periods. On the whole we Deep sleep during the first half of the night, and REM sleep during the second. Deep sleep and REM sleep are divided up into shorter sleep periods which alternate. So now we can list the characteristics of Deep sleep and of REM sleep, as follows: Deep SleepDeep sleep appeared at about the time warm blooded mammals evolved from their cold-blooded reptilian ancestors by developing the ability to maintain a constant body temperature by biological processes. As we progress from being awake through sleeping to being awake again, the frequency of the brain waves drops, reaching its lowest point while in Deep sleep and then rises again to the wide-awake level. (See Figure 1 `Sleep Pattern: Day - Night - Day`) During Deep Sleep the body`s muscles are relaxed and heart beat and breathing are slow and regular. Deep sleep `dream-like experiences are more like ordinary everyday thoughts and are usually rather banal and repetitive in content`. During Deep sleep `one is not dreaming but thinking.` REM Sleep (Rapid-Eye-Movement sleep)REM sleep also appeared at about the time warm blooded mammals evolved from their cold-blooded reptilian ancestors by developing the ability to maintain a constant level of body temperature by biological processes. At this constant level there is a small but closely controlled body temperature rhythm (we tend to go to sleep after our body temperature has began to fall and tend to wake up after it has started to rise) and the body-temperature clock also controls the appearance of REM sleep. On the whole we REM sleep during the second half of the night, after Deep sleep and before waking up through relaxing to being fully awake. In REM sleep the body`s muscles are paralysed while heart-beat and breathing fluctuate as they would during emotional upsets in waking life. Brain waves look like the waking pattern. The eyes move rapidly and continuously. <7> Persistent rapid eye movement shows that dreaming is taking place and the brain paralyses the sleeper so that the dreams cannot be acted out. Dreams tend to consist of "sensory illusions or hallucinated dramas" (imagined feelings or awarenesses), are not usually remembered unless the dreamer wakes up from the dream itself. "The length of time taken to dream of certain events is about the same as the time it would take to experience those events in waking reality." Project index
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