When you are awake and engaged in some kind of activity (walking, reading, etc.), your brain is activity processing external sensory input from the environment, which, together with your, memory provides the raw material from which you construct a model a model of the world. While awake and active, the model accurately reflects your relationship to the external world. if you are awake but physically inactive, the balance of input moves from the external to the internal.
To a certain extent your thinking becomes independent of external stimuli, your mind wanders, you daydream. With part of your mind you are modeling words that might be, rather than the current actual environment. Still, you tend to maintain a reduced model of the external world and your attention can easily be drawn back to it, if, for example, some sign of danger appears.
In the case of sleep, so little sensory input is available from the outside world that you stop, maintaining a conscious model ot it. When your sleeping brain is activated enough to construct a world model in your consciousness, the model is mostly independent from what is happening in your environment- in other words, a dream. The sleeping brain isn't always creating a multi dimensional world model. Sometimes it seems to be merely thinking, or doing very little.
The differences in mental activity during sleep depend largely upon differences in the state of the sleeper's brain. Sleep is not a uniform state of passive withdrawal from the world, as scientists thought until the twentieth century. There are two distinct kinds of sleep: a quite phase and an active phase, which are distinguished by many differences in biochemistry, physiology, psychology and behaviour.
Changes in the brain waves ((electrical activity measured at the scalp ), eye movements, and muscle tone are used to define the two states. The quite phase fits fairly well with the commonsense view of sleep as a state of restful inactivity - your mind does little while you breathe slowly and deeply; your metabolic rate is at a minimum, and growth hormones are released facilitating restorative processes.
When awakened from this state, people feel, disoriented and rarely remember dreaming. You can observe this state in your cat or dog, when it is quietly sleeping in a moderately relaxed posture (in the case of cats, the " sphinx" posture) and breathing slowly and regularly. Incidentally, this is the phase of sleep in which sleep-talking and sleepwalking occur. The transition from quite to active sleep is quite dramatic. During the active sleep phase, commonly called rapid eye movement or REM sleep, your eyes move rapidly about (under closed lids of course), much as they would if you were awake.
Your breathing become quick and irregular, your brain burns as much fuel as it does when you're awake, and you dream vividly. If you're male, you probably will have an erection; if you're female, increased vaginal blood flow. While all this activity is happening in your brain , your body remains almost completely still (except for small twitches), because it is temporarily paralyzed during REM sleep to prevent you from acting out your dreams.
The "sleep paralysis" of REM sleep doesn't always turn off immediately upon awakening; this is why you may have experienced waking up and not being able to move for a minute. Sleep paralysis can seem a terrifying experience, but actually it is quite harmless, and indeed, can even be useful for including lucid dreams.
You can get a good view of "paradoxical sleep," as REM sleep is called in Europe , when you see your cat or dog sleeping totally collapsed , breathing irregularly, twitching, showing eye movement s, and in the case of dogs. tail wagging, whimpering, growling, and barking. This is when people justifiably say, " Look Spotto is dreaming!"
Reference: Exploring the World of Lucid Dreaming : Stephen LaBerge, Ph.D & Howard Rheingold
No right click