Why do we dream?
The events of dreams are often impossible, or unlikely to occur, in physical reality: they are also outside the control of the dreamer. The exception to this is known as lucid dreaming, in which dreamers realize that they are dreaming, and are sometimes capable of changing their dream environment and controlling various aspects of the dream. The dream environment is often much more realistic in a lucid dream, and the senses heightened.
There is no universally agreed-upon biological definition of dreaming. General observation shows that dreams are strongly associated with REM sleep. REM sleep is the state of sleep in which brain activity is most like wakefulness, which is why many researchers believe this is when dreams are strongest, although it could also mean that this is a state from which dreams are most easily remembered. During a typical lifespan, a human spends a total of about six years dreaming (which is about 2 hours each night). It is unknown where in the brain dreams originate - if there is such a single location or why dreams occur at all.