Nightmares are disturbing dreams that can cause feelings of fear, anxiety, or sadness. They often occur during the REM sleep stage, when most dreaming happens. Nightmares can be triggered by stress, trauma, or certain medications, and they may involve frightening scenarios or situations. People of all ages can experience nightmares, but they are more common in children. While occasional nightmares are normal, frequent nightmares can disrupt sleep and affect daily life. In some cases, therapy or relaxation techniques may help reduce the occurrence of nightmares and improve sleep quality.