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. When to see a doctor Occasional nightmares aren't usually a cause for concern. If your child has nightmares, you can simply mention them at a routine well-child exam. However, consult your doctor if nightmares: 
Occur frequently and persist over time
Routinely disrupt sleep
Cause fear of going to sleep
Cause daytime behavior problems or difficulty functioning
