Interrupted Memories: Alcohol-Induced Blackouts

What Are Blackouts?

Blackouts are gaps in recall caused by impairments in memory consolidation while intoxicated. They happen when a person drinks enough alcohol to prevent a brain area called the hippocampus from forming new memories. Without a functioning hippocampus, a gap occurs in the brain’s record-keeping system and, when the alcohol wears off, the person cannot recall what transpired.

Types of Blackouts

There are two types of blackouts; they are defined by the severity of the memory impairment. The most common type is called a “fragmentary blackout” and is characterized by spotty memories for events, with “islands” of memories separated by missing periods of time in between. This type is sometimes referred to as a grayout or a brownout.

Complete amnesia, often spanning hours, is known as an “en bloc” blackout. With this severe form of blackout, memories of events do not form and typically cannot be recovered. It is as if the events simply never occurred.

When Do Blackouts Occur?

Blackouts tend to begin at blood alcohol concentrations (BACs) of about 0.16 percent (nearly twice the legal driving limit) and higher. At these BACs, most cognitive abilities (e.g., impulse control, attention, judgment, and decision-making) are significantly impaired. The level of impairment that occurs at such high BACs makes the intoxication level associated with blackouts especially dangerous.

Blackouts can occur at much lower BACs in people who drink and take sleep and anti-anxiety medications.

Research indicates that blackouts are more likely to occur when alcohol enters the bloodstream quickly, causing the BAC to rise rapidly. This could happen if someone drinks on an empty stomach or consumes excessive amounts of alcohol in a short amount of time. Because females, on average, weigh less than males and pound for pound, have less water in their bodies, they tend to reach higher peak BAC levels than males with each drink and do so more quickly. This helps explain why being female appears to be a risk factor for having blackouts.

Because blackouts tend to occur at high BACs, they commonly stem from binge drinking, defined as a pattern of drinking that increases a person’s BAC to 0.08 percent or higher. This typically occurs after 4 drinks for women and 5 drinks for men—in about 2 hours. In fact, many people who have blackouts do so after engaging in a behavior known as high-intensity drinking, which is defined as drinking at levels that are at least twice as high as the binge-drinking thresholds for women and men.