A nightmare is an unpleasant dream. It usually manifests with feelings of fear or horror, but can also manifest despair, anxiety or sadness. A person having a nightmare will usually awaken even from a deep sleep and will often suffer a bout of insomnia as a result.
In the vast majority of cases, nightmares do not indicate any pathology and are perfectly normal. They may manifest themselves with minor pathologies such as uncomfortable sleep positions, fever, stress or anxiety. They can also be the result of digestive problems, particularly having a large meal before sleep.
Frequent or recurring nightmares can cause ongoing insomnia and may require medical help to deal with the underlying cause, which is frequently psychological. However, they can be the result of both physical illness and difficult to treat mental illnesses such as post-traumatic stress disorder.
Treatment can focus on dealing with underlying disorders, or teaching the patient techniques to deal with the nightmare, including psychotherapy and desensitization.
See also night terror