Most dreams occur during REM sleep and typically last 20-30 minutes, though some may be shorter or longer. Nightmares, which cause strong negative emotions like fear, mostly consist of stresses from when one is awake and are most common among young children. Lucid dreaming is when a person is aware they are dreaming and may have some or full control over the dream. Theories suggest dreams allow the brain to make connections between experiences and emotions, though the exact reasons we dream remain unknown.