A dream is the experience of envisioned images, sounds, or other sensations during sleep. It occurs in humans, most mammals, and some birds. The events of dreams are often impossible or unlikely to occur in physical reality, and are usually outside the control of the dreamer. The exception is lucid dreaming, in which dreamers realize that they are dreaming, and are sometimes capable of changing their oneiric reality and controlling various aspects of the dream, in which the suspension of disbelief is often broken. Dreamers may experience strong emotions while dreaming. Frightening or upsetting dreams are referred to as nightmares. The discipline of dream research is oneirology.
There is no universally agreed-upon biological definition of dreaming. General observation shows that dreams are 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.
More at
http://en.wikipedia.org/wiki/Dream.
