Dream

Dreams are what a person sees and hears in their mind when they are sleeping. They are often similar to real life in some ways, but can also be very strange. Dreams can be so real, the person dreaming may not believe they are awake. This tends to happen during a false awakening, where a person dreams of waking up. A person has around 4 - 6 dreams per night, and tend to only remember the last two.
Sometimes a person realizes during a dream that they are dreaming, but keeps having the dream. This is called a lucid dream. This happens very little for most people, but for some people it happens often. During a lucid dream, a person usually has full control over their body, as well as the environment around them. There are no limits in lucid dreams, sometimes not even the imagination.
Most people remember their dreams in some way or another, even if it is only a small part, but children are very likely to remember most of their dream clearly. It is often easier for people to remember dreams if they write down what happened in the dream just after they wake up. Because of this, many people have dream diaries where they describe each dream they have. This is called dream recall, and the more it is done the better they remember their dreams. If a person is woken up during a dream, they tend to have a much more reliable memory of it.
Nightmares are dreams which scare or shock people. Nightmares are usually based around that person's everyday fears, like spiders or dark places, but even a dream that's not about those things can feel unpleasant. Nightmares are caused by many different things: being uncomfortable or in pain while sleeping, sickness, stress, or even eating right before sleeping.
There are many different theories about why people dream and what their dreams mean. Every person has different dreams. Some psychologists believe that dreams reflect what is happening in the unconscious mind (the part of the mind that works by itself). Others think that people, places, and objects in dreams are symbols for other things in the dreamer's real life. Throughout history people have tried to make sense of dreams to learn things from them, and have often used them for divination or fortune-telling. Today there are still many books and websites devoted to making sense of dreams.
Ancient ideas on dreams
Generally speaking, ancient civilisations thought dreams were messages from the gods (see the works of Homer) or alternatively some kind of prophecy. However, they knew that often dreams misled the dreamer, and invented various explanations for this. Aristotle started off with ideas like this, but later became more skeptical, and denied the divine origin of dreams.
Freudian theory
In his Interpretation of Dreams, Sigmund Freud connected them to his ideas on psychotherapy. Dreams, in Freud's view, are forms of "wish fulfillment". They are attempts by the unconscious to resolve a conflict of some sort. Because the information in the unconscious is in an unruly and often disturbing form, a "censor" in the preconscious will not allow it to pass unaltered into the conscious. Freud describes three main types of dreams: 1. Direct prophecies received in the dream; 2. The foretelling of a future event; 3. The symbolic dream, which requires interpretation.
Some authors, such as Hans Eysenck, have argued that the dreams Freud cites do not really support his theories. Eysenck argues that Freud's examples actually disprove his dream theory.
Modern work
Since Freud, the emphasis has been on the biology of dreaming.
The discovery of REM (rapid eye movement) and non-REM sleep has been important. Researchers have done many studies on this. Subjects have been woken up in both stages and asked what they were thinking about. It is clear that the reports from non-REM stages were different from REM stages. In particular, dreams occur mostly when the brain is in the REM state. There is also some relationship between dreaming and daydreams. Both seem to occur in a cycle of 90110 minutes.
Apparently, "there is no evidence that a more useful understanding of personality can be gained from them than can be divined from the realities of waking behaviour".
If sleep is prevented, people suffer and get worse at every kind of waking activity. From this it is clear that one important function of sleep is to maintain normal brain activity during awake time. Somehow, during sleep the brain gets restored to its normal functioning. Sleep is, so far as is known, universal amongst vertebrates. That also argues for its great importance. However, it is not known whether dreaming supports this repair function of sleep, or whether it is something which just happens.
References
Psychology
VOA Special English words
Consciousness
Sleep