Dreams are images or mental activity that occurs during sleep. You can dream in any stage of sleep, but your most vivid dreams usually occur in REM sleep.
Scientists aren't sure why humans dream in the first place, but they think it has something to do with memory. Dreaming may help the brain get rid of any unnecessary information or memories while processing and storing what is important. Some people feel refreshed after sleeping and dreaming, even if they do not remember the dream. People are more likely to remember the last dream they had during their sleep cycle.