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Nightmares are frightening dreams that occur later in the sleep period and cause a person to awaken with a lingering sense of fear or anxiety. They are lengthy sequences during which a person dreams of imminent physical danger (e.g., pursuit, attack, injury). In some cases, dreams involve personal failure or embarrassment. Nightmares may occur at night or during the day while napping.

Sleep is divided into five stages: stages 1 through 4, which range from the lightest sleep to deep sleep, and the fifth stage, which is known as rapid eye movement (REM) sleep. During a typical night’s sleep, a person cycles through these stages approximately every 90 minutes.

Dreams and nightmares occur during REM sleep. During this stage, breathing becomes more rapid, irregular and shallow and the eyes jerk rapidly. People experience longer cycles of REM sleep more frequently as the night progresses. This intensifies dreams in the second half of the night, and increases the likelihood of nightmares later in the night or close to morning.

Nightmares may occur after a traumatic event and mimic aspects of the trauma. However, most nightmares do not recount actual events. Some patients may have multiple nightmares with a similar theme during the night. People who have post-traumatic stress disorder (PTSD) frequently have nightmares related to the stressful event they have experienced, such as combat, abuse or disaster (e.g., fire, hurricane). Children commonly have nightmares about real or fantasy creatures, such as animals, insects or monsters.

After waking up, patients usually have a lingering sense of fear or anxiety and can often remember the nightmare in detail. Nightmares do not usually cause significant distress, but in some cases may cause disruption in daytime functioning. This is particularly true if nightmares are frequent and sleep is avoided. Patients who avoid sleep may experience excessive sleepiness, poor concentration, depression, anxiety or irritability. In addition, recent research suggests that nightmares may be associated with suicidality.

According to the American Academy of Family Physicians, nightmares affect 20 to 39 percent of children aged 5 to 12. Among adults, 5 to 8 percent experience ongoing problems with nightmares.

According to the American Psychiatric Association, 10 to 50 percent of children aged 3 to 5 have nightmares that are intense enough to wake their parents. As many as 50 percent of adults report having occasional nightmares.

More women than men report having nightmares. However, it is not known whether women experience nightmares more often than men or whether the discrepancy reflects an increased tendency among women to report nightmares.