Lucid dreaming happens
1) when a person is at the "deep sleep" stage.
2) during the "Rapid Eye Movement" stage.
3) when a person is not completely asleep.
4) when a person is having a nightmare.
When you fall asleep, you enter a strange and magical world. This is the world of dreams.
In Greek and Roman times, it was believed that gods communicated with people through dreams and that dreams could predict the future. Often these dreams were difficult to understand, so a person known as an oracle would be consulted to interpret the dream. The oracles that could correctly understand the dreams were highly valued since they advised what needed to be done or avoided in the future.
In the 20th century psychologists such as Sigmund Freud developed the theory that dreams express our deepest fears and desires. Therefore, common dreams such as failing a test or being late may simply be an expression of what you are afraid of, while dreams of becoming a millionaire may just express what we wish. However, some dreams might not be so obvious and can be cloaked in symbols that only a trained specialist can interpret. This is actually similar to the oracle mentioned above, only instead of predicting the future the psychoanalyst explained the psychological meaning of a dream.
According to the latest scientific research, dreams are simply random signals that are interpreted by the brain. When we sleep, some parts of the brain are activated, such as the visual centres of the brain, while other parts are deactivated, like those that are responsible for logic, which is why even though dreams are so strange, we don't question them or understand that we are dreaming. A consequence of this theory is that dreams are simply mixed up memories, fears and worries and therefore don't need to be interpreted to be understood.
Some people claim that they don't dream at all. Scientific research has proven this false. When we dream, our eyes move rapidly beneath our closed eyelids. This is known as Rapid Eye Movement or REM. Even animals dream, some even more than humans. Dolphins dream the least, while opossums dream the most. The reason some people don't remember that they were dreaming is that following REM sleep is another stage called deep sleep, where we don't dream. During this stage dreams are forgotten. If people wake up during REM sleep, they remember having dreamt. If they wake up during the stage of deep sleep, they may have the illusion that they did not dream at all.
A new kind of dreaming that has recently been studied is called lucid dreaming. This is a dream where you understand that you are dreaming and can control it. Normally when you are dreaming, the part of the brain responsible for logic is turned off. Somehow during a lucid dream, these logical centres are activated and a person is half-awake and half-dreaming. One of the benefits of lucid dreaming is being able to avoid a nightmare by realizing that it is just a dream. Another positive outcome of lucid dreams is that they are very fun. One of the first things people usually try in a lucid dream is flying, and the thrill of exploring a dream and being able to control it is an unforgettable experience.
Somehow during a lucid dream, these logical centres are activated and a person is half-awake and half-dreaming.