sentence: Nightmares are common, but nightmares that happen very often can cause problems with sleep.
sentences_before: A 'nightmare' is a very powerful dream that the sleeper finds disturbing. Nightmares usually have either physiological causes, like a high fever, or psychological ones, like trauma or stress in the sleeper's life.
sentences_after: They may cause people to get medical help.
concept_name: nightmare
headings: Nightmare
categories: Psychology