Nightmares are vivid dreams that contain frightening images or cause negative feelings such as fear, terror, and/or extreme anxiety. If awakened from REM sleep during a nightmare, the sleeper can usually provide a detailed description of the dream content. The nightmare can cause the sufferer to awaken in a heightened state of distress, resulting in perspiration and an elevated heart rate. Often it takes time to recover from the negative emotions invoked by the nightmare and the person may have difficulty returning to sleep.

Nightmares may be able to be distinguished from sleep terrors based on the timing of the episode and the whether dream content can be recalled. Nightmares are more likely to occur during the last third of the night when there is a higher concentration of REM sleep.

Nightmares have been known to have both psychological and physical causes. Factors including illness, anxiety, or even sleeping in an uncomfortable position can lead to bad dreams. Post traumatic stress disorder can trigger frequent nightmares, as can side effects from various medications and narcotics such as amphetamines and cocaine.

There are a number of methods, both medical and non-medical, to alleviate the conditions that often cause chronic nightmares. Often, alleviating the stresses in the home or personal life will reduce anxiety at bedtime that can trigger nightmares. Regular exercise, yoga and meditation may also help reduce stress and improve sleep quality.