Blackouts are periods of unconsciousness or memory loss. Generally, a blackout is described as a period of unconsciousness or lack of awareness when you are unable to recall what happened or what you did. Blackouts may occur as a result of brain damage, drug side effects, or disorders affecting brain function, such as epilepsy. Fainting, also known as syncope, is a term used to refer to a blackout. Conditions that can cause syncope include cardiac arrhythmias (abnormal heart rate or rhythm), abnormalities of the heart muscle or valves, or a condition called postural hypotension, in which a person faints after standing up quickly from a lying position and there is insufficient blood flow to the brain at that point.