Two types of sleep
When a person is sleeping, there are moments of non-REM and REM (Rapid Eye Movement) sleep.
REM is where the eyes move rapidly beneath the eyelids. It has been believed that people only dream during REM sleep, but it was discovered that REM and dreams are two independent entities as they are located in different locations in the brain.
REM occurs during deep sleep stages and dreams which take place during this period are more vivid. Nightmares are also believed to take place during REM sleep. Some medications and medicines such as anti-depressants are said to stop REM sleep; however, it is still possible for dreams to take place.