Theory:

     When we wake up, our temperature and blood pressure rise to normal. Our heartbeat and breathing also become normal and we are fully awake, and have forgotten most, if not all, the dreams that we had while sleeping.
 
     What is a dream? It is an activity of the mind that takes place when we are asleep. Some dreams are probable while others are not. That only means that many of the things that happen in dreams could happen when we are awake. Others could not. Dreams seem to be important for several reasons. One is that a dream can help us to sleep through noise or other disturbances. For example, the alarm clock rings, but our mind causes us to dream that the telephone or doorbell is ringing, and that we are awake and answering it.
Explanation:
 
As seen previously, sleep goes in a circle of stages ranging between light sleep to deep sleep, followed by REM sleep. Most of us regain our consciousness while we are in the REM state and lose it when we sail back to stage two of the cycle.
 
As we wake up, our body would get back to its normal mode. Our temperature, blood pressure, heartbeat, and breathing turn normal. By the time we are fully awake, we would have forgotten most of our dreams.
 
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Our body turns normal and most of our dreams get forgotten as we wake up
 
We have heard a lot about dreams. So, what is a dream?

Dream is a series of events, thoughts, images, and sensations that happen in our mind when we are sleeping. Fundamentally, it is an activity of mind. It is almost like a movie that we watch in our mind. However, the events may not be correlated or may even seem very strange. While some of the dreams are probable, others are not.
 
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Dream is a mental activity where anything becomes possible

For instance, you may dream of getting a pet you like. This is an example of a probable dream because the situation is likely to happen. Or you may have a dream of meeting a person you admire. This is also a case of probable dream because the situation is not entirely impossible.
 
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'A cat dreams of eating fish' - an example of probable dream
 
But imagine you have a dream where your kitten suddenly turns into a beautiful person or a gigantic dog? That is unlikely to happen, and hence it is an improbable dream.
 
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'A boy on a moon-swing' - an example of improbable dream

Dreams are essential for various reasons. It can help us sleep through the night without disturbances. For instance, imagine your alarm ringing when you are dreaming. Your dream would take the sound from the alarm and will interpret it as a telephone or doorbell. You would also have a dream of waking up and attending to the same. Hence, dreams would help you sleep through disturbances.
 
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Dream can help us to sleep through noise or other disturbances

Meaning of difficult word:
 
S.No
Words
Meanings
1
ProbableLikely to happen or be the case
Reference:
National Council of Educational Research and Training (2006). The Wonder Called Sleep. (pp. 25-27). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.