Nightmares represent our innermost fears.. The monsters and other terrifying images that appear in these dreams are believed to come from shared mythology recognised by all cultures. The word nightmare itself comes from gaelic mythology in association with the horse goddess Rhiannon. Rhiannon is connected with with dreams and the moon deity, as symbolised by the white horse. Rhiannon was certainly a mightmareish creature, as she was accused of eating her own son, Pryderi.

Bad dreams can have physical causes. if your body comes under stress, due to high temperature for example, it can cause hallucinations as you sleep. Nightmares can also induce sleep walking by increasing the flow of adrenaline and producing the 'flight or fight' response in the sleeper.