You're more likely to have a nightmare in the second half of your night. Nightmares may occur rarely or more frequently, even several times a night. Episodes are generally brief, but they cause you to awaken, and returning to sleep can be difficult.

A nightmare may involve these features:

• Your dream seems vivid and real and is very upsetting, often becoming more disturbing as the dream unfolds
• Your dream storyline is usually related to threats to safety or survival, but it can have other disturbing themes
• Your dream awakens you
• You feel scared, anxious, angry, sad or disgusted as a result of your dream
• You feel sweaty or have a pounding heartbeat while in bed
• You can think clearly upon awakening and can recall details of your dream
• Your dream causes distress that keeps you from falling back to sleep easily

Nightmares are only considered a disorder if you experience:

• Frequent occurrences
• Major distress or impairment during the day, such as anxiety or persistent fear, or bedtime anxiety about having another nightmare
• Problems with concentration or memory, or you can't stop thinking about images from your dreams
• Daytime sleepiness, fatigue or low energy
• Problems functioning at work or school or in social situations
• Behavior problems related to bedtime or fear of the dark

Having a child with nightmare disorder can cause significant sleep disturbance and distress for parents or caregivers.

Nightmare disorder is referred to by doctors as a parasomnia — a type of sleep disorder that involves undesirable experiences that occur while you're falling asleep, during sleep or when you're waking up. Nightmares usually occur during the stage of sleep known as rapid eye movement (REM). The exact cause of nightmares is not known.

Nightmares can be triggered by many factors, including:

• Stress or anxiety. Sometimes the ordinary stresses of daily life, such as a problem at home or school, trigger nightmares. A major change, such as a move or the death of a loved one, can have the same effect. Experiencing anxiety is associated with a greater risk of nightmares.
• Trauma. Nightmares are common after an accident, injury, physical or sexual abuse, or other traumatic event. Nightmares are common in people who have post-traumatic stress disorder (PTSD).
• Sleep deprivation. Changes in your schedule that cause irregular sleeping and waking times or that interrupt or reduce the amount of sleep can increase your risk of having nightmares. Insomnia is associated with an increased risk of nightmares.
• Medications. Some drugs — including certain antidepressants, blood pressure medications, beta blockers, and drugs used to treat Parkinson's disease or to help stop smoking — can trigger nightmares.
• Substance abuse. Alcohol and recreational drug use or withdrawal can trigger nightmares.
• Other disorders. Depression and other mental health disorders may be linked to nightmares. Nightmares can happen along with some medical conditions, such as heart disease or cancer. Having other sleep disorders that interfere with adequate sleep can be associated with having nightmares.
• Scary books and movies. For some people, reading scary books or watching frightening movies, especially before bed, can be associated with nightmares.

Nightmares are more common when family members have a history of nightmares or other sleep parasomnias, such as talking during sleep.

Nightmare disorder may cause:

• Excessive daytime sleepiness, which can lead to difficulties at school or work, or problems with everyday tasks, such as driving and concentrating
• Problems with mood, such as depression or anxiety from dreams that continue to bother you
• Resistance to going to bed or to sleep for fear you'll have another bad dream
• Suicidal thoughts or suicide attempts