Nightmares are distressing dreams that often cause feelings of fear, anxiety, or sadness. They typically occur during the REM sleep stage, when most dreaming happens. Common themes in nightmares include being chased, falling, or experiencing loss. These dreams can disrupt sleep and lead to feelings of fatigue during the day. Nightmares can affect people of all ages, but they are particularly common in children. Factors such as stress, trauma, or certain medications can increase the likelihood of experiencing nightmares. While occasional nightmares are normal, frequent occurrences may indicate underlying issues that could benefit from professional attention.